It is important to note here that Jacob is weak in his faith. Like, really really weak.
He WANTS TO believe. He is quite outspoken about his faith and brags about it but just looking at his behaviour makes it clear that his faith is not on a strong foundation. He has gone back and forth in his faith, he has gone back and forth in his psychotic delusions about setting up a new king of Britain. All his talk about how he plans to convert others relies on them changing fate just as easily as he does. "Just tell 'em that X is the one true faith/king/whatever and they'll start to worship it/him/me" is pretty much all he knows about converting people. And it worked on him. All his conversions from one faith or king to another happened just because he googled something and went with the first result that came up.