The great St. Patrick was once an atheist
St. Patrick's Confession is full of interesting information about his family, his youth, and a near-apostolate-destroying scandal that turned his fellow bishops against him.
I can hear the howls of outrage. But before you grab torches and pitchforks and head over to my house, give me a moment. Obviously, Patrick was not an atheist when he began his world-changing missionary work. Yet by his own admission, there was a time in his life when his likelihood of ever being venerated as a saint was nil.