In his work heaven and hell, he associated the religious epiphany and the psychadelic experince of mescalin/lsd. To quote directly from "The Doors of Perception" pg. 73 of the perennial library edition:

All I am suggesting is that the mescalin experience is what the catholic theologians call "a gratuitous grace", not necessary to salvation, but potentially helpful.

He stated this because of the similarities of the molecular structure of mescalin to certain (bio types correct, or expound upon this statement) types of adrenalin.