That was what I was always trying to get at with the people who identified as former believers: wanting to know what was the basis of their belief, had they encountered God, and if so, how could they then turn around and say it did not happen? They came to believe they had somehow just imagined it, that it wasn't real, they fooled themselves into belief etc, when in one case especially it seemed that the person eventually renounced their faith because God did not keep proving His existence to them... He did not answer their prayers they way they wanted Him to. They felt they were wasting their timeNot possible.
A true Christian was/is saved by God. A person can't just pretend it never happened.
It would be like un-knowing something, or un-seeing something. Can't be done.