Though I wasn't actually raised religiously, just the prevalence of religion in society led to many of my early theories have religious components.

I thought that between dying and being reborn you spent your time in heaven as an angel. While there, you could select your next parents from all the currently pregnant people on earth. I imagined giant 'rooms' full of computers in heaven, where every pregnant woman had a profile you could check out (mind, that was in the early 90s - I like to think I secretly invented social networking^^). Once you decided on a mother, you'd be kicked down into her womb and be reborn. I did realize pretty soon after that this theory doesn't hold water when you see that many children are not born into happy families, but for a short while I was certain I picked my mother from all the possible mothers around the world. She was very charmed by that :D