As I said in Just Believe, it can be hard to believe in someone or something when they have done nothing to warrant your trust or your belief in them.
However, it can be easier to believe in someone when the way they act or behave, confirms the belief. i.e. When someone professing to be a Christian, acts and behaves as you would expect them to be behave.
When you see someone’s actions reflect their beliefs, it makes us more likely to believe in them.