I have been thinking about this a lot recently. Apparently, people who go on LSD trips come back with this idea that there's "no separation" between individuals. It's such a consistent report that it's hard to just dismiss out of hand, but I think the genesis of it is actually far more mundane than the situation would imply.
Apparently, from my understanding, what LSD does is it randomizes how individual neurons, and therefore, brain systems interact. It breaks the normal bonds, the perceptual filters, and, I'm told, biases that exist within individuals. It "opens the gates of perception" and remakes you into your own true essence.
I think, honestly, this may be exactly what happens, but LSD also disrupts your ability to assign relevance and importance to things, and it destroys your discrtionary filter (which new agey types call the "Ego", and it's an apt label since it DOES honestly also tend to lead you away from uncomfortable truths), so it may be extremely difficult for an indifi cual to really parse their experience.
What I believe is really happening is that it's giving people access to their own psyche at the lowest possible level, it's giving them a backdoor into their own "debugger's console", the raw operation of their own information processing system, and since they lack the knowledge of infromation thereoy, they can't really comprehend what they've seen, even thoough their minds seem to comprehend the processes by which this information has been sorted. So what does the human mind do with information that seems to make sense despite being beyond their ucrrent logical faculties? Well, the answer is obvious. You say it was a religious experience.
I think that people with even a little bit of information theory training, maybe even some programming training, could get a lot more out of an LSD trip than people who just go in raw. They would have the base discretion to understand what the things they're incorporating into their conscious mind actually mean.
At the end of the day, the human mind isn't magical. We may have someything like a "soul" but it fors NOT come from some deitiy. We are what emerged as a consequence of the laws of rality, not transcendent of them.
So, to go back to the initila conceit of this post, the perception of "unity" is actually exactly what these people think it is-- but the way they says it implies both more and less, because they don't have the benefit of previous training in objective theory-- they are describing a phenomenon from a subjective point of view, when it CAN also be described (semi)objectively.
Basically, they have realized that, when you take "me" out of the equation in your thought processes, you are of the same stuff as those around you-- you are a being, a pattern, existing in the material world, and subject to it just as much as any other. The same way there is no separation between the engine and the car that it drives, or the road and the car, there is no separation between people. We are cause-and-effect, we are the pattern, we are what we make ourselves become.
Full disclosure: The writer has never done any kind of "mind-expanding" drugs. Only dumb crap like weed and alchyhol.