Most atheists including those who were previously religious are nonbelievers because they have used reason and logic to reach the conclusion that there's no evidence for the existence of god(s) and therefore theism makes no sense . Yet, there are others who lost their faith in a supreme being due to disappointment that their prayers went unanswered or because of some affliction that beset them or their loved ones. In other words, the motivation for their non-belief is based on emotion and projection (narcissism?): "God let me down; therefore he must not exist."
So the question is: how sincere and committed are such former believers to atheism? Not very, I would wager. It seems to me that genuine atheism is based on objective considerations, not subjective feelings. In other words, those who arrived at their non-religious perspective through discontent with a supreme being's "performance" may not really be atheists at all, but just mad at God and may be prone to regain their religious faith if he makes things right again in their eyes. (I also explored the "mad at God" concept in my post "What's God Got To Do With It? Fallacies of Theistic Belief" )
Some atheists-by-reason may assert that it doesn't matter how one became a non-believer or why (s)he rejects a supernatural explanation for the existence of the universe. As a beleaguered and perhaps the most hated minority in America, we need all the adherents we can get. And besides, there's no membership committee, vetting process, or test to determine one's dedication to atheism anyway. This is a tempting argument, but I personally I would rather have a "lean and mean" core of members in the atheist movement who arrived at their convictions of godlessness through due intellectual diligence. Those who claim to be non-believers merely as the result of discouraging circumstances vis a vis the relationship with their god may just be "foul weather" atheists who revert to theism when the sun shines again in their lives.