One book series that was extremely successful in putting the Christian faith into at least a few characters while still keeping very much in genre was the Dresden Files. Michael Carpenter is a Scion of his faith, and he exudes it in this fantasy world setting, while not being preachy (except when necessary to the main character). Yes, it's fantasy, yes it's an alternate world, and yes, it's just one or two characters, but it's still very obviously a Christian faith character and it's very well done.

It can be done, it just might take a little finesse.