I'm not religious in the slightest and wouldn't have considered myself spiritual, but these last few years I hit the strong stuff pretty frequently and have wavered quite a bit on my acceptance of the ineffable. The sensation to me is more akin to tapping into quantum realms / states / streams of information that aren't quite yet explained or proven, and are a combination of *perhaps* external factors but also the operation of the largely unexplained human brain, an organ which could already loosely be decribed as an organic multidimensional quantum computer.
Problems for the non-religious and non-'spiritual' skeptic in me began to occur when inscrutable, difficult to explain experiences began piling up. Instead of saying I'm now spiritual, however, I'm content to believe that, as indicated above, these experiences are largely a function of aspects of the human brain both known and unknown, with perhaps a dose of external phenomena also unknown to science but copacetic more or less with various paths of theoretical quantum physics.
Some observations from the deeper dives: the brain creates one's reality to a higher degree than I would have ever expected. It's all fabricated in the mind, from the interpretation of the senses into the experience we accept as seen / heard / smelled / touched reality to the way our brain creates the narrative of our lives and assembles the narrative threads around us. This doesn't mean that the perceived reality of the sober brain isn't more or less there and isn't essentially what we experience: that the table is there, someone's knocking at the door, this piece of fruit is rotten, that this person with the blonde hair and red sweater is being socially rude, that the landlord is conspiring to evict you; what it does mean is that our experience of these things is entirely built in the mind, like an ultra-complex video-game render with a virtual novelist rapidly interpreting then transcribing the story.
Once drugs have shown one this potential of the mind it's difficult not to crash the metaphysical into science.
I'm academically interested in a variety of spiritual practices now, but I understand that none of them are strictly true, my interest is instead in finding the overlap between beliefs and squicking out the universal truths of the human experience. In these overlaps one can interpret a sort of hidden service manual for the human experience, like one might for a washing machine or television.
edit: to be clear, these weren't insights that happened during trips in the form of intellectual or spiritual epiphanies, but post trip reactions to drugs and drug combinations completely altering my experience of reality, clear as day, as well as drug-induced psychotic breaks that led my brain to fabricate my perception of narrative reality in ways that persisted far after the drugs wore off but were clearly 'false' in hindsight.