Question: "What does the Bible say about an out of body experience / astral projection?"
Answer: Information about the "out-of-body” experience is both vast and subjective. According to Wikipedia, one out of ten people claims to have had an out-of-body experience (OBE), and there are many different types of the experiences claimed. They range from involuntary out-of-body experiences or near-death experiences that happen after or during a trauma or accident, to what is called “astral projection" in which a person voluntarily tries to leave his/her body behind and ascend to a spiritual plane where he/she believes he/she will find truth and clarity.
A few famous Christians have had what might be called, in today’s world, an out-of-body experience, most notably the Apostle Paul. He says in 2 Corinthians 12:1-4, "I must go on boasting. Although there is nothing to be gained, I will go on to visions and revelations from the Lord. I know a man in Christ who fourteen years ago was caught up to the third heaven. Whether it was in the body or out of the body I do not know—God knows. And I know that this man—whether in the body or apart from the body I do not know, but God knows—was caught up to paradise. He heard inexpressible things, things that man is not permitted to tell." In the verses preceding this passage, Paul lists his "boasts" or the things that, if he were counting on works and good deeds to secure his salvation, would get him into heaven. Though he seems to be referring to a third party, scholars agree that he is speaking of himself in the third person. Therefore, he is including this apparent out-of-body experience in his list of boasts. The point he is making is that any revelation that comes from outside the Bible (extra-biblical revelation) is not a reliable source, and as Paul says, "There is nothing to be gained by it." This does not mean that his out-of-body experience wasn't real, only that he is not relying on it to give him truth or really to benefit himself or other people in any way.
An involuntary out-of-body experience or a near-death experience, like the Apostle Paul's, should be treated in the same way as a dream in the life of a Christian—an unexplained phenomenon that may make a good story, but does not give us truth. The only place we find absolute truth is in the Word of God. All other sources are merely subjective human accounts or interpretations based on what we can discover with our finite minds. The book of Revelation, or John's vision, is an exception to this, as are the prophecies or visions of the Old Testament prophets. In each of those cases, the prophets were told that this was a revelation from the Lord, and they should share what they had seen because it was directly from the mouth of God.
A voluntary out-of-body experience, or an “astral projection,” is a different story. A person trying to achieve an out-of-body experience in order to connect with spirits or the spirit world is practicing the occult. There are two forms of this. The first is called the “phasing” model, in which the person tries to find new spiritual truth by accessing a part of the mind that is "shut off" during everyday life. This practice is connected to Buddhism or postmodernism and the belief that enlightenment is achieved from looking within oneself. The other form, called the “mystical” model, is when the person tries to exit the body entirely, his/her spirit traveling to another plane that is not connected to the physical world at all.
The Bible explicitly warns against occult practice, or sorcery, in Galatians 5:19-20, saying that those who practice it will not inherit God's kingdom. God's commands are always for our good, and He commands us to stay far away from occult practices because there is great potential, when trying to access the spiritual world, of opening oneself up to demons who can tell us lies about God and confuse our minds. In Job 4:12-21, Eliphaz describes being visited by a lying spirit in a vision that tells him God does not regard humans and that He doesn't care for us, which is false! The phasing model is also futile, according to Scripture. Jeremiah 17:9 says, "The heart is deceitful above all things and desperately sick; who can understand it?" and 1 Corinthians 2:1-5 says, "When I came to you, brothers, I did not come with eloquence or superior wisdom as I proclaimed to you the testimony about God. For I resolved to know nothing while I was with you except Jesus Christ and him crucified. I came to you in weakness and fear, and with much trembling. My message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirit's power, so that your faith might not rest on men's wisdom, but on God's power." It is futile to search for infinite wisdom inside the finite mind of man.
One concrete example of this comes from the popular book 90 Minutes in Heaven by Pastor Don Piper. Piper describes what is, in essence, an out-of-body experience he had after a severe car accident, during which he believes he died and went to heaven for ninety minutes. Whether or not Piper did actually see heaven or spend time there is debatable, and in the end nobody but God knows. However, there is a serious problem, theologically speaking, with the conclusion Pastor Piper draws from his experience. He tells the reader that, now that he has "been to heaven," he can speak comfort to grieving people at funerals "with more authority" than he could previously. Piper's motives are correct: he wants to give people hope. However, it is dead wrong to say that his own subjective experience will give him more authority to administer the hope of heaven than the perfect truth of Scripture would do.
In conclusion, whatever sort of out-of-body experience we are talking about, the main point to remember is that an out-of-body experience will give us neither truth nor knowledge. If an involuntary out-of-body experience occurs in the life of a Christian, the best approach would be to consider it in the same category as a dream—interesting, perhaps, but not a source of truth. Christians are to find truth only in the words of God, as Jesus prays in John 17:17, "Sanctify them in the truth; Your word is truth."