User blog comment:Blaziken rjcf/Priest in "O Brother, Where Art Thou?"/@comment-1975828-20160531085413/@comment-1975828-20160531092506

I guess the priest was so full of faith and believing in the protection of God, that he did not concern himself with the possibility that one day, God's greatest enemy or polar opposite would walk into his church. He looks pretty old too, so I guess the lack of any real threats has boosted his faith and, like you said, made him naive.

Supposedly, Amara hoped to gain further knowledge about her brother's religion, which doesn't seem to have worked out, I guess. As for Castiel, he was very broken at the time they met, and only had one use: being a messenger. Not to mention, Amara had just swallowed Ambriel's grace and knowledge, so whatever Ambriel saw in Castiel, Amara saw too.

This is mostly just speculation. :/