Okay, so here’s an article from The Atlantic as a primer (I’m going to try and spare you the usual anti-apologetics stuff I get from various atheist YouTube channels).
What Happens to the Brain During Spiritual Experiences?
As you say, all the evidence we have points to the “mind” as being resident solely in the brain, and these intense spiritual experiences, while seemingly unique and profound and so far removed from anything (some people) would think could possibly be resident in the brain… are just that. They produce observable phenomena, repeatable in multiple studies. That Buddhists or Christians or Muslims or Jains or whoever might insist, with absolute certainty (as if absolute certainty is warranted) that their experience must have involved some supernatural transcendence to another plane of blah blah blah does not make it so.
Yes, the religious are often quite good at appearing certain. So what? I could sit across from you and tell you how certain I am that the world is flat, keep a totally flat expression on my face, and insist over and over again, with great fervor that I am correct and anyone who disagrees is either lying or deceived. But that wouldn’t make it so. I would hope you’d see through that too, and walk away from such a conversation without the slightest bit of doubt as to the world’s roundness.
Should someone insisting, however fervently, that there is no self convince you that they are right? Should it even be sufficient to give you cause to doubt? I say no.
To the extent that there could possibly be any basis for the notion that there is no self, I would say it must surely depend on a twisting of the very definition of the word to make it so. Like people who define god as “the universe” and conclude that since the universe exists, god exists (and not even in some sort of prime mover/cosmological argument sense, but strictly in the sense that they have defined god to be something other than what the bulk of the population would ever understand god to be, and concluded that since that thing—it might as well be a coffee cup—exists, “god exists.”)
There’s also this video of a lecture given at the Royal Institution (UK) about neuroscience and consciousness which touches on some issues relating to our understanding of consciousness and the mind or “self.”
The Neuroscience of Consciousness – with Anil Seth
And finally (also from the Royal Institution), there’s The Science of Psychedelics - with Michael Pollan, which talks about, among other things, how certain drugs can cause experiences very similar to the sort of spiritual or mystical experiences reported by people who practice meditation. Around the ten minute mark he relates a story about an atheist who, under the influence of drugs, described feeling as if they were “bathed in god’s love.” As an atheist… (and they remained an atheist after, the point is there’s that feeling).
Each video is about and hour long (hey, if this has been bothering you for a while, what’s a couple hours?) and the speakers are doctors and scientists who are interested in the truth (lower case), not spiritual gurus who want to be admired for their “certainty.”
Bottom line: Do not let those who pedal in certainties and absolutes brainwash you into thinking that you do not exist or that there is no “self.”