A very interesting topic for exploration! I’ve read accounts of trascendental experiences during religious rites such as prayer or meditation (one book I read on meditation, the author described a sensation of the crown chakra exploding with light that connected him to the universe, which strikes me as extremely compelling to consider regardless of one’s personal faith or lack thereof!).
Perhaps the religious/spiritual part is merely the vehicle with which a person reaches a state of pure present centredness and peace. I’m still on the fence about God in the religious sense, but can’t deny that energy runs through all matter, call it Tao, Buddha Nature, God, or whatever else, I think it’s all the same stuff essentially. Though I consider organised religion various cultures’ method of reaching a universal truth so perhaps I’m not the most objective.
I find it fascinating though that the brain physically changes during such an experience :)