Not OP but as an atheist for me spiritualism was made concrete when after a meditation session I intuitively knew and felt that myself and the world were the same. I watched at some familiar trees through my windows and felt I and them were the same and actually existed. This sensation brought me great joy and calmness. And then it passed, I was myself again, and the world was again separate from me.

This was some two decades ago. I've never felt anything like that after that, but the memory of the sensation is still very strong.

No drugs. Just the most basic 'lie-down-relaxed-and-watch-at-a-dot' exercise. And wham.

It was pretty cool. I was an adolescent back then and I had decided several years before that logically religion was total BS. Had I been religious I'm sure I would have interpreted my experience as being in direct communion with God. But, I interpreted it as a neurological response to my meditation - which did not make the experience any less spiritual.

The experience did not reveal anything new to me, but it made me feel the truth that we all are one and connected, philosophically speaking.

Feeling is something purely logical discourse seldom provides. This experience was pure feeling. Like, I intuitively feel my legs are part of me. I felt the whole world was part of me. Now, this is at the same time true - or - false. We are made of the same atoms and are interconnected through our actions and the laws of physics. At the same time, it's a bit silly to describe oneself extending beyond ones body. So, one can choose was I enlightened by a fundamental truth or was I just a bit silly after a brief session of neurohacking.