When a religious person encounters an atheist who used to be a devout follower of the same religion, a response that is all too commonly heard is that the current atheist never used to be a believer. Complete rubbish in almost every case, it’s hardly impossible for a theist to become an atheist or vice versa.
Not that I always believe it when a theist claims to have been an atheist in the past. I would take it at their word if it was just that, but often enough the claim is accompanied by “I was angry at God”, which completely demolishes the credibility: it is a little hard to be so angry at something one does not even think exists. I put slightly more credence to claims for someone to have converted to a religion out of intellectual reasons, except the claims almost invariably undermine themselves as the intellectual arguments put forward tend to be really bad, not even passing the “the same sort of reasoning would not apply to another contradictory religion” filter. There have been exceptions, although the religious concepts have been so watered down that they may as well not have been religions.
Without extraneous and dubious claims as the above, I would usually take a theist’s claim that they used to be an atheist at their word. Oddly enough, it’s much rarer to see a current atheist with a dubious claim to having previously been a theist. It might be because I’m biased …