Father David Gideon was a pastor in a town that was supposedly full of demons.
History
Season 5
Gideon was a normal guy with a daughter, until one day, the "angels" spoke to Leah and told about the Apocalypse. He and the town began to defend the town against the demons. Despite the town being war zone, he still married people, and among preaching to them. David begins noticing that his daughter is acting weird and asks her to stop driving people to kill others. "Leah" threatens him to let her go or she'll name him next, giving him a sign that "Leah" is not Leah. Then Castiel grabs him and brings him to Sam and Dean who ask him to kill Leah (who in reality is the Whore of Babylon). He first disagrees, and agrees to do it. Although he is not the one who deals the final blow.