It is a sin if one practices "spiritual arts" that are not taught in the sc<x>riptures.
The term Astral Projection isn't really true. There are four sheaths of energy that surrounds Soul. The Physical, Astral, Causal, Etheric. Somtimes, referred to as the Physical, Emotional, Memory, Intellect/Subconscious. Each live in a Field of Consciousness. When you desire something...your Astral Body or Field of Consciousness moves the physical to that which is desired. When you Astral Travel you let go of your Physical Body (Consciousness) and enter into a different Field of Consciousness. In this casé...the Astral Body which resides in the Astral Plane or Field Of Consciousness. To Astral project...you simply switch your Awareness from one plane.to another. Most do this everyone night when they sleep, but not aware of it. Many call the Astral Plane the Dream Worlds. When you day dream...you are doing Astral Projection. To many make a big real out of Astral Projection which is as natural as imaging. Its almost a sin not to become aware of these different Fields of Consciousness that surround Soul. Beyond Astral Projection is Soul Travel. just like the Physical is limited to. Particular Field of Light...the Astral Body is as well. You are Soul...a Unit of Awareness. Becoming aware of that which makes up the whole should be a goal. If you want to learn more about Soul Travel...I highly reccommend Eckankar.org. I have been doing Soul Travel for over 40 years...What a God Blessing it Is. If you seek or want to know the Truth...you won't know it until you experience it first hand. Otherwise...you live a life of HOPE instead of a State of Knowing. You don't have to die to visit the God Worlds as they exist within you. There's nothing to fear accept the Truth. You are Soul and exist because God loved You.
I think it can be dangerous. The Bible talks about demon possession and if a house is swept clean, then a demon can take possession. If you, your spirit, travels out of your body you are leaving it open for possible demon possession. That's why I gave it up when I became a believer.
@neeki: That's part of it too. Opening the mind leaves one vulnerable to unclean spirits and demons as well.
No its just a fairy tale
It is not a sin. There is a book called Astral Breathing I reccomend.