History is replete with religiously minded folks that went off the deep end. Just to name a few from the last 50 or 60 years, we could note David Koresh and the Branch Davidians; Bonnie Nettles and Marshall Applewhite of the Heaven's Gate cult; Bishop James Pike and Bishop John Shelby Spong (although, arguably Spong kept his sanity) of the Episcopal Church; and Jim Jones of the Peoples Temple and the Jonestown massacre. There are likely good reasons for such unravellings of mind and soul that happen when folks seek to open themselves to spiritual realities.
The first would be a naive assumption that spirit is better than matter and that all things truly spiritual are equally good and benevolent and helpful. Or similarly, that things spiritual can be approached without any danger to the person seeking a spiritual experience or “enlightenment” or secret knowledge or to touch that which is truly transcendent. The Bible is pretty clear that there are spiritual forces of wickedness that are quite powerful (powers and principalities in the heavenlies) and that masquerade even as angels of light (pretending to be the “good guys” all the while) seeking to devour unsuspecting souls. And then there is the bigger truth – God (the One True God) is a consuming fire. If you try to lay hands on that fire in the wrong way, you might get more than burned. Think, for instance, of the movie Indiana Jones and the Raiders of the Lost Ark when some of the bad guys pulled the lid off the Ark of the Covenant and looked inside (I know, that's just Hollywood nonsense, but how about 1 Samuel 5 for the real deal...).
But perhaps a more subtle temptation is to try to recreate an amazing spiritual experience in which the glory of God was beheld, where His nearness and majesty and holiness captivated body, mind and soul and the person felt they were in Heaven itself. Some of us have had just such an experience, and it has changed everything. We know what awaits us when we pass from this life, and we know the One we serve in this life.
Other folks don't want to wait for the consummation of all things when Christ returns. Some folks don't want to wait until the Kingdom of God comes in full. They want to hurry up and get to the Beatific Vision, to Union with God. But trying to get there in our as-yet-not-glorified humanity can be a deadly pursuit.
Consider the experience at Mt Sinai of Moses, Joshua, Aaron, Nadab, Abihu and 70 elders of Israel (Exodus 24:1-11). These all went partway up Mt Sinai and “they saw the God of Israel. There was under his feet as it were a pavement of sapphire stone, like the very heavens for clearness.” They “beheld God, and ate and drank.” Amazing, isn't it? Talk about a spiritual experience!
Moses got summoned to go up to God higher in the mountain, and Moses' assistant Joshua (yes, that Joshua) accompanied him. The others went back down to the congregation of Israel and waited for Moses to return. That return didn't happen for another 40 days, and the people grew restless and thought something might have happened to him. That's where things began to fall apart.
So.... Of those that went up on Sinai and saw the God of Israel, Aaron made a golden calf and led the people into false worship. Nadab and Abihu offered “profane” or unauthorized fire before the Lord and were consumed (died when fire went out from the Lord and consumed them). And the 70 elders are likely those who later prophesied when the Lord took some of the Spirit that was on Moses and put it on those who were appointed to help Moses. But these 70 did not remain in the same fear and trust of the Lord as Moses. They did not trust the Lord when the spies were sent out and they, like almost their entire generation, died in the wilderness because of their unbelief. So close, but yet so far...
The Catholic fiction writer Walker Percy likened the transcending of world and human relations (through such things as a religious experience of God) as being like a satellite newly put in orbit. The person with the transcending experience of the divine realizes that their true place is not really in this world but with God (however “God” might be perceived and understood by said person). Because they no longer feel at home in the world, they wish to remain in orbit around whatever it is they have experienced. But they can't sustain their orbit, and inevitably come back to earth. Some make the transition well and could be said to be faithful to the vision they have seen. Others “burn up” as it were on re-entry. Something breaks in their psyche and they crash back to earth, spending the rest of their days trying to recreate that fleeting moment of religious exaltation (that spiritual high), and possess what can never be possessed in that way.
I suppose the bottom line is this: If the experience isn't of the risen Lord Jesus, or if the experience doesn't lead you to Jesus, watch out! Pursuing something spiritual which isn't of God can lead you away from the light of truth and into the darkness of a really dark night a long long way from Home. And if the experience is of God, beware trying to possess His glory for yourself on your terms and for your purposes. You might get consumed in the process.