Board Thread:General Discussion/@comment-25359252-20130628122522/@comment-10862555-20130628212750

If he worships Talos but doesn't fight with the Stormcloaks, he could have a vendetta against him. That would give him a personal reason to fight and make it a personal goal. Being very devout in the Aedra he may believe that Tiber Septim's empire is meant to last.

You could be a monk who was unwilling to continue letting the Stormcloaks sully the name of Talos' Empire. While on a pilgrimage from the monastery to an Imperial outpost to enlist, a Thalmor Justiciar spotted an Amulet of Talos (Among the others) in your satchel. You were sentenced to death for the crime of Talos worship and sent to Helgen with the Imperials you were traveling to join.