The parts of the brain that light up.
¯\_(ツ
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Several possible explanations...
All spiritual experiences are merely based in the brain.
Genuine experience and "false" experience share the same brain functions
All religions are equal spiritual paths
etc.
#1 seems likely because as you said before parts of the brain do light up when so-called spiritual experiences happen.
#2 is possible but unproven.
#3 is also possible but unproven. Different religions make factual claims that contradict each other and many claim a monopoly on a spiritual path.
Based on #3 you will have to agree that just because you have a so-called spiritual experience involving a certain religion does not makes the supernatural claims it makes true.
Have you considered the possibility that "spiritual" experience is simply a unique form of emotional experience and this explains why it activates different parts of the brain than normal emotional experiences.
I argue that spiritual experience are emotional because they involve a feeling of a burning in the chest, feelings of heighted happiness and exitement, feeling of knowing or discovery, ideas popping in the head, a feeling of self-knowledge, a feeling of peace and holiness, a feeling of unconditional love and compassion for others, or a feeling of oneness with the universe or presence of a higher being.
What is a religious experience to you and how does it differ from a emotional expierence by the sensations, thoughts and experiences it creates.