high doses of mescaline or mushrooms, and of course dmt. i find most thing natural tend to be more more spiritual in nature. (it might be me and not the substance) but usually any Psychedelic in high doses an bring about a very religious/spiritual experiance.
Mescaline for me as well. The dreaminess and beauty of a strong cactus trip has yet to be matched by any other entheogen. Oddly enough, morning glories have yielded the most profound revelations, though in brief flashes.