The spread of New Age teachings is hardly surprising - The Bible warns that "in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons" (1 Timothy 4:1).
Free Dictionary defines channeling (once called spiritism) as
However, while the concept remains the same, there seem to be two types of channelers. The first one resembles the 'gypsy at the fair' idea inasmuch as many people, grieving the loss of a loved one, attempt to communicate with their spirit through the medium. This practice became especially popular after World War I when so many died in the war and were now supposedly on the 'other side' - as it is termed. Others consult the spirit world to try to see the future or get advice on something they are contemplating doing - a possible romance, a business endeavour, or even mere curiosity.
The second type of channeler is very well known among New Age devotees. The person concerned may or may not have actively sought out the spirit world, but were somehow exposed to a psychic or supernatural phenomena. For example,
Sanaya Roman began to experiment with a Ouija board and eventually became a channel for an entity called Orin.
Tam Mossman who did some editorial work done on Jane Robert's "Seth" books. When, one day, he pretended that Seth was taking over his body, but was himself taken over and thereafter channelled an entity called 'James'. 
When she was fifteen years old, Alice Bailey was approached by a mysterious entity who wore a turban and called himself 'Kuthumi'. One thing led to another and she wound up writing some 24 books, 19 of which were literally dictated by an ectoplasmic apparition who called himself * Djwhal Khul (The Tibetan), who claimed he lived in a physical body, on the borders of Tibet. See Alice Bailey and The Lucis Trust
These spirits or that actively seek to transmit information are highly evolved spirit non-physical entities often simply referred to as "The Masters" or "The Ascended Masters".. They are reputed to be spiritually enlightened beings who, in past incarnations, were mere mortals, but who have undergone a series of spiritual transformations called initiations. These beings communicate through their human channelers in a variety of ways - often using the medium's own vocal cords to do so. Douglas James Mahr describes what happens to J. Z. Knight, one of the most popular New Age channelers, when she begins to channel an entity that calls itself Ramtha:
"When Ramtha begins his appearance in the embodiment of J. Z. Knight, a completely different sensation from that of J. Z.'s embodiment is felt. The body of J. Z. Knight is still present but a totally different personality emerges-- her body seems larger and stronger, bursting at the seams; the softness of J. Z.'s mannerisms and facial expressions are replaced by those of a man; body postures and gestures surging with power; his concentration becomes an intensity, the voice is that of another knowingness." 
Other forms of channeling include automatic writing, visions, and inner voice dictation all of which are claimed to greatly benefit not only individuals, but mankind as a whole. See Footnote 1
According to various New Age directories there are any number of organizations that offer courses and workshops designed to teach others to channel. And, of course, there are thousands of channelers operating throughout America some of whom have, for want of a better expression, hit the big time. Alice Bailey being one of the most influential. She founded the Lucis Trust (formerly Lucifer Publishing Co.) which enjoys "Consultative Status" with the United Nations and has not only spawned several other organizations such as the Arcane School, Triangles, World Goodwill, Lucis Publishing, Lucis Productions, Lucis Trust Libraries, the New Group of World Servers, but Lucis Trust also maintains the UN meditation room.
But here is what I find extremely interesting.
Almost the entire New Age Movement is based on information and instructions given by these spirit guides. They have dictated their messages or indulged in philosophical exchanges - all of which have been duly written down by their compliant minions, and then presented to the world as New Revelation, without one ounce of evidence that the spirit guide was who he claimed to be, or that his message could, in any way be trusted.
They speak and human beings fall over like ninepins, rushing to embrace whatever religious philosophy is being propagated. It is accepted that these spirits are who they claim to be, and that the particulars they provide are unreservedly true accepted without reservation or question. No further proof or evidence asked for, and none received. These people are making an effort to shape the very world on the words of a spook without credentials or any form of evidence, that they are who they say they are.
And they call Christians fools? Seems to me the boot is on the other foot.
See Choose Life That You May Live
Many, if not most, non-Christians assume that Christianity is a "blind faith"... that Christians ignore reality and have unquestioning loyalty to an absolute belief system without proof or evidence. In fact, that they believe contrary to all evidence and facts. Much to the contrary, the Christian faith is a commitment based on evidence. The Judeo-Christian faith consistently stresses the importance of truth, and makes appeal to evidence to support it's truth claims. This evidence includes the Bible’s humanly impossible authorship, fulfilled prophecy, and it's archaeological and scientific accuracy... none of which are seen in the books of other religions.
The few other examples given below are just a very small drop in a very big bucket.
Edgar Cayce, also known as "The Sleeping Prophet", appeared to be a kind and humble man motivated by the desire to help his fellows. He claimed to be a Christian and read the Bible from cover to cover once a year. However, his readings had much more in common with New Age and occult philosophy than Biblical Christianity. For example, reincarnation is explicitly affirmed in the Edgar Cayce readings.
Edgar Cayce's A.R.E. (Association for Research and Enlightenment) says he had
the ability to put himself into a sleep-like state by lying down on a couch, closing his eyes, and folding his hands over his stomach. In this state of relaxation and meditation, he was able to place his mind in contact with all time and space — the universal consciousness, also known as the super-conscious mind. From there, he could respond to questions as broad as, "What are the secrets of the universe?" and "What is my purpose in life?" to as specific as, "What can I do to help my arthritis?" and "How were the pyramids of Egypt built? His responses to these questions came to be called "readings," and their insights offer practical help and advice to individuals even today. 
One of the two sources his information came from was the so-called "Akashic Records," which is apparently some kind of universal database for every thought, word, or deed that has ever transpired in the earth. A nonprofit organization, the Association for Research and Enlightenment was founded to facilitate the study of Cayce's work. See Edgar Cayce
Jane Roberts: According to the Seth learning center,
"Seth is the internationally acclaimed spiritual teacher who spoke through the author Jane Roberts while she was in trance, and coined the phrase "You Create Your Own Reality." Seth's empowering message literally launched the New Age movement. The books written by Seth have sold over seven million copies and have been translated into over eleven languages. 
J.Z. Knight: 71 year old J.Z. Knight - American New Age teacher and author has been in business for a long time. She is known for supposedly channeling Ramtha - an ascended Master Teacher who lived on earth as a human 35,000 years ago. During his lifetime, he is said to have "achieved the supreme convergence of spirit and matter and became an ascended master" whose "powerful message of hope for humanity has already changed countless lives" 
She has appeared on US television shows including Larry King and The Merv Griffin Show, and has attracted attention from several celebrities. J.Z. lives in Yelm in Washington state where she is president of Ramtha's School of Enlightenment. and hosts UFO symposiums. In 2007, she was invited by the United Nations Staff Recreational Council to speak to the Society for Enlightenment and Transformation (SEAT) 
Jach Pursel: Since 1974 Jach Pursel has channelled the entity that calls himself 'Lazaris' who, in friendship and love, has generated a" remarkable body of tools, techniques, processes, and pathways for our Spiritual Journey". To say nothing of creating "sacred talismans". Pursel also founded Concept: Synergy, the organization that oversees all Lazaris events and offerings.. 
Sanaya Roman has been channeling Orin, "a wise and gentle spirit teacher, for many years, bringing through Orin's wisdom via books, audio courses, etc. As said on her web site
"All of Orin's work is to assist you in unfolding your potential, finding your own inner wisdom, and in growing spiritually... Orin calls the path he offers a Path of Self-Realization". 
Shawn Randal channels the being called Torah and runs what seems to be rather profitable channeling classes in which you
"connect mentally and physically with the beings of Light and love you have agreed to know" . Torah and Shawn "are dedicated to the evolution and development of consciousness on both personal and global levels". 10]
Helen Schucman: is a familiar name in many Christian households thanks to apostate leaders like Robert Schuller who actually sold her book A Course in Miracles in the Crystal Cathedral bookstore. It is estimated that at least five million people have been exposed to the teachings in the Course
However, what seems to have escaped so many Christians is that A Course in Miracles is a channelled work that claims to be a revelation from Jesus Christ Himself.
In fact, prominent New Ager Marianne Williamson's million-copy bestseller, A Return to Love: Reflections on the Principles of A Course in Miracles (Harper-Collins, 1992) is, as the name suggests, heavily based on the Course. Oprah Winfrey was so enthralled with this book that she bought a thousand copies for her friends and others. Influential psychiatrist Gerald Jampolsky extolled its virtues in his books and lectures appearing on shows like the Phil Donahue Show, Today, and 60 Minutes. See A Course in Miracles
What The Bible says
It is apparent that ever since man existed, he has been trying to get in touch with unseen worlds. The Bible confirms that there are 'rulers', 'powers', 'spiritual forces' - all of them evil.
For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places. (Ephesians 6:12 NASB)
Spiritists and mediums must have been so common among the pagan people in the area that, in the book of Deuteronomy, the Lord warned the Israelites that when they entered the land that the Lord had given them, they were not to imitate the "detestable" practices of those nations. It was these practices that would cause them to be driven out of the land. His instructions to the Jews were very clear.
"There shall not be found among you anyone who makes his son or his daughter pass through the fire, one who uses divination, one who practices witchcraft, or one who interprets omens, or a sorcerer, or one who casts a spell, or a medium, or a spiritist, or one who calls up the dead. "For whoever does these things is detestable to the Lord; and because of these detestable things the Lord your God will drive them out before you. "You shall be blameless before the Lord your God. "For those nations, which you shall dispossess, listen to those who practice witchcraft and to diviners, but as for you, the Lord your God has not allowed you to do so. (Deuteronomy 18:10-14 NASB)
In fact, consulting a medium warranted the death penalty
'Now a man or a woman who is a medium or a spiritist shall surely be put to death. They shall be stoned with stones, their bloodguiltiness is upon them.'" (Leviticus 20:27 NASB)
Manasseh was one of the worst kings of Judah. Among his many evils, he "...made his sons pass through the fire in the valley of Ben-hinnom; and he practiced witchcraft, used divination, practiced sorcery and dealt with mediums and spiritists. He did much evil in the sight of the Lord, provoking Him to anger. (2 Chronicles 33:6 NASB)
Yet, King Saul did so when he felt backed into a corner.
Saul, Samuel, and The Witch of Endor
When Saul saw the host of the Philistines gathered for war, he was very afraid, and enquired of the Lord. However, probably because Saul had already fallen out of favour, the Lord did not answer him. Saul then turned to the witch of Endor and told her that he wished to consult with a spirit. Not just any spirit, but the spirit of Samuel the prophet.
Then Saul disguised himself by putting on other clothes, and went, he and two men with him, and they came to the woman by night; and he said, "Conjure up for me, please, and bring up for me whom I shall name to you." ... Then the woman said, "Whom shall I bring up for you?" And he said, "Bring up Samuel for me." .... The king said to her, "Do not be afraid; but what do you see?" And the woman said to Saul, "I see a divine being coming up out of the earth." ... Then Samuel said to Saul, "Why have you disturbed me by bringing me up?"..." (1 Samuel 28:8, 11, 13, 15 NASB)
Note the words "bring up", or "come up" are often used in the account because Sheol is said to be beneath us. See What and Where is Hell? Part I... Sheol And Hades Also note that Samuel sounded positively peeved when he was summoned and asked why Saul had 'disquieted' or 'troubled' him? (1 Samuel 28:15).
Verse 12 says that "When the woman saw Samuel, she cried out with a loud voice; and the woman spoke to Saul, saying, "Why have you deceived me? For you are Saul." Her words make it very clear that the woman was utterly unprepared to actually see Samuel. Perhaps she expected to see something she had encountered before (a familiar perhaps). A second possibility was that she had previously used tricks to fool people into believing she was a medium and Samuel's very real appearance startled and frightened her.
The fact that Samuel did appear probably led her to recognize Saul - because it is unlikely that the prophet would have answered the call of anyone less than the king (although they became adversaries when Saul fell from the straight and narrow).
And the result of Saul's disobedience?
So Saul died for his trespass which he committed against the Lord, because of the word of the Lord which he did not keep; and also because he asked counsel of a medium, making inquiry of it, and did not inquire of the Lord . Therefore He killed him and turned the kingdom to David the son of Jesse. (1 Chronicles 10:13-14 NASB)
Christians will commonly say that Saul was not communicating with Samuel himself, but a demon disguised as Samuel. However, the text repeatedly identifies the spirit as Samuel (Verses 12, 14, 15-16). Besides which, no demon would have been able to so accurately prophesy what Saul's fate would be the very next day, and tell Saul the exact reason this would happen (because he failed to obey the Lord and destroy Amalek). Infallible and accurate prophecies about the future is a prerogative reserved for the Supreme Being alone.
See God and His Bible. The Reliability of The Old Testament - Part 4 of Choose Life That You Might Live
Although extremely rare, the dead are apparently occasionally permitted by God to communicate with those still alive on earth. Remember that Elijah and Moses appeared to Jesus and some of his disciples (Matthew 17) However, these incidents do not sanction man's attempt to communicate with the dead.
The Bible's Warning
As mentioned, Leviticus 20:27 is very clear that consulting a medium warranted the death penalty. And nothing has changed in the New, except that judgment is not immediate. The Father has given us from now to the day we die to heed His warnings - one of which is 'testing' the spirits
Beloved, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; and every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world. (1 John 4:1-3 NASB)
Channeled spirits who are usually considered "good" certainly do not confess that Jesus Christ came in the flesh from God. Much to the contrary, they usually claim that what the Bible teaches is false. important Christian doctrines are usually contradicted or denied. For example, Seth said "There is no personal God-individual in Christian terms."  and "You are God - You create your own reality"
Psychic Sylvia Browne's spirit guide Francine, is the main source of information for Browne's Journey of the Soul Series.
Browne denies the pantheistic belief that all is God, but she nonetheless holds that we are a “divine spark” that emanated from God and that everyone has his or her own "God center." God, moreover, does not punish or judge, and there is no hell; man made up hell. It does not matter if Jesus is the Son of God because everyone is. According to Browne, Jesus did not die on the cross but came to bring wisdom. See I See Dead People by Marcia Montenegro
And there is more - plenty more.. For more contradictions between what the Bible says and what the spirits say See The Teaching Of The Spirits
In other words, the most likely scenario is that the spirits with whom Sanaya Roman, Jane Roberts, and Helen Schucman claim to communicate with are demonic.
Footnote I - The Coming Age of Aquarius and The Reappearance of The Christ
The many headed hydra called the New Age has, over the last couple of decades, become even more sinister with every passing year. Most New Agers share a common vision ... the present phase of human existence is one of transition from the Piscean Age of rationality, to the Aquarian Age of spirituality, and harmony of all things. [See New Age Overview ]
That the New Age is within our reach may not, in itself, sound too bad.
However, what is particularly alarming is that while many New Agers may merely subscribe to some elements of New Age philosophies, A large branch of the New Age movement believes that the utopian age will only come about under the leadership and guidance of a world leader/teacher. They contend that Buddha, Muhammad, Confucius, Jesus, and many others were "christs", but one greater than all of them - a "World Teacher" will soon come to usher in the New Age.
They believe and teach that the world has to be prepared for this one-world leader who is called by various names including ‘The Christ’, ‘Sanat Kumara’ or ‘Maitreya’, and who will, in human form, begin preparing humanity for the advent of the Age of Aquarius... a glorious New Age millennium. He will save the world from sliding into chaos by teaching us to live at peace and unity with each other. Many believe that this leader, possibly a man who has achieved godhood, will possess unparalleled wisdom and knowledge and perhaps psychic abilities to boot. As said by Warren Smith
According to the "new gospel, "Christ is not a person. It is an office. The "new gospel" teaches that while Jesus of Nazareth occupied the office during His active ministry, He no longer holds that same position. Today the office of "Christ" is occupied by someone else. And this "someone else" is presently in the process of establishing contact with humanity. This "Christ" intimates that he is already in the world awaiting mankind's call.
Concerned that humanity is in peril and facing possible extinction, this "Christ" explains that his "new gospel" will unify the world's major religions and bring peace to the world 
In other words, one the main goals of the New Age movement has become a concerted effort to facilitate what is called "The Reappearance of The Christ". With this ultimate goal in mind, an incredibly huge network of groups, trusts, foundations, clubs, religious groups etc. have become focused on making changes not only at an individual, but global level.
Not only do these organizations present what they call "ageless wisdom" teachings, but they are doing everything they can to create the conditions necessary for ‘The Reappearance of The Christ’. Heralded as the 'salvation of earth' or the 'new earth', this movement has gathered immense size and power along the way, and is poised to come to fruition. See The Global Alliance
 Information on Sanaya Roman and Tam Mossman is from Who Are the Channelers by Dr. John Ankerberg and Dr.John Weldon. https://www.jashow.org/articles/new-age/who-are-the-channelers/
 Ramtha with Douglas James Mahr, Voyage to the New World (New York: Fawcett, 1985), p. 17.As quoted in What is the New Age movement? by Kenneth R. Wade. https://www.ministrymagazine.org/archive/1989/03/what-is-the-new-age-movement
 Who Was Edgar Cayce? Twentieth Century Psychic and Medical Clairvoyant. https://www.edgarcayce.org/edgar-cayce/his-life/
 Seth Learning Center. Seth: The Spiritual Teacher That Launched The New Age. https://www.sethlearningcenter.org/
 JZ Knight.com. About JZ Knight. http://www.jzknight.com/about/
 JZ Knight.com. http://www.jzknight.com/2007/05/
 The Lazaris Material. About Us https://lazaris01.worldsecuresystems.com/about
 About Sanaya, Orin, and Thaddeus. Greetings from Sanaya. https://www.orindaben.com/pages/home/aboutsr/
 Classes and Privates. http://www.shawnrandall.com/index_files/Page18.htm
 What Torah And Shawn Do. http://www.shawnrandall.com/index_files/Page6.htm
 Jane Roberts. The Seth Material: The Spiritual Teacher that Launched the New Age Paperback – New Awareness Network; 1 edition (January 1, 2010). Pg. 269
 Warren Smith. Reinventing Jesus Christ: The New Gospel. Preface. Pg 9