How common is it for atheists to become believers in God?

I don't know what the numbers are exactly. I used to argue with an atheist on a regular basis in delphi forums. The argument came to a halt when I exposed to him that even though he had no faith in a god, he still had faith of some kind (namely in his own logic). I came back to the forum about a year later and found that he became a Mormon. I thought that was particularly weird.