It also depends on what type of church you are playing for (there really is no standard list).
Practice playing through popular chord progressions so that when you hear them, you'll be able to "fake" your way through them. For instance: Playing in COGIC, you can get anything thrown at you at anytime.
For years I spent all my time learning songs but, couldn't play any song that I hadn't learned ahead of time. I used to wonder how people remembered all these songs. It wasn't till years later that found out that they were playing progressions and not songs (if you know what I mean). Being self taught can be a drag sometimes.
Don't get me wrong, there are some songs that you have to know but, the majority of them, if you can recognize the changes you'll be fine.