I'd like to jump in here and give my personal experience along these lines.
I was a hateful, Christian persecuting teenager who happened to have a believing sister. She would once in a while ask me to go to her youth group with her, and I would always become violently angry with her, to the point of one time lifting her off the floor by her throat and telling her if she ever mentioned God to me again I would kill her. She never said another word to me about Him again.
My fear of all things having to do with God was amazing and horrible at the same time. When I found myself at an occasional funeral or wedding I would break into a sweat at the mention of Jesus. I hated God, I hated anyone who talked of God, and I never stopped to ask myself why. I only knew that I had to flee or lash out at the mention of Him.
One day when I was 18 I went into my sister's bedroom to get a record I had loaned her and on the way out, while passing her dresser, my eyes fell on her bible. At that moment I was COMPELLED to pick it up. I remember wondering what that THING was doing in my hands and as I found myself laying on my bed reading about this Jesus whom I had blasphemed and hated, I began to believe.
Our loving heavenly father can, and apparently sometimes will directly compel someone to believe, as He did so graciously with me.
I can only assume that He has dealt with others similarly.
Thanks for bearing with a story that arguably belongs in a personal testimony thread.