Do you think the altering of our brain chemistry is permanent? Are they only during the experience? Or does the brain eventually return to normal?
The answer to this question would depend on how exactly you define a drug's alteration of our brain.
The short answer is that the drug has a finite half-life. It binds to receptors while it is present in the body, and stops having a psychedelic action after it leaves our body.
But the down-chain effects of it's binding--for example the networks of neurons involved in storing memories of the experience, or the triggered release of BDNF and NGF which cause new neural connections to grow, or the down-regulation of 5ht receptors in response to the chemical, are all lasting artifacts of it's presence in our brains.
For example, there's been a study that showed that, in drug-naive individuals, a SINGLE mushroom experience could cause lasting changes in the expression of the openness trait on personality tests. These changes were shown to stay true at (I believe) 6 months and 1 year later.
Before I mentioned that a drug-induced spiritual experience could be equally as legitimate as a natural spiritual experience. I think this is where that comes into play. If, for example, you take a trip to a poor country, and are deeply emotionally affected by the destitution and suffering you see around you, the knowledge of that suffering, and the lasting effects of that emotional impact, could be felt much later. Even a year later, after you're comfortable back at home... After your eyes are no longer sending that emotionally displeasing stimulus to your brain, you will be a different person for having experienced it. This is because the neurons involved in your emotional response to the situation, by activating, have created down-stream changes in the connections between other neurons in your brain. You have a painful memory. You have new associations--perhaps, for example, you now associate a certain food or a certain word with your memory of that experience and the emotions it evoked.
So even though it was a temporary experience, it has permanently changed you. I believe the same to be true for psychedelic experiences.