only to discover he was great at training, a good swordsman, but unable to hurt a fly
village was attacked by pillagers and he stood frozen... (villlage is ok and family is alive and thriving!), but he blames himself for not being able to help (that's when he unknowingly swears an oath to protect those in need)
he thinks himself a simple swordsman, but he has attracted a "spirit" that lives out of the bonds that oaths make. (Find familiar :) This spirit can't talk but kinda "teaches" him the ways of the light, and he blames the magic he can do on it (while it's the other way around)
he thinks poorly of himself, thinks he is a coward for not being brave enough to kill when reason says that killing that guy/monster/thing would avoid more danger
that's why he's not constantly preaching over others, he's actually jealous they can do what he cannot