Philosophy is divided into various topics, one of which is Philosophy of Religion; arguments for and against the existence of God. During the crusades when my ancestors were hacking peoples heads off, Christian and Islamic scholars got together and shared ideas, theories and knowledge, a fair amount of which actually came from the Muslim side. Do philosophy.
Other sections include Epistemology: the study of knowledge (rationalist or empiricist) probably one of the oldest debates in philosophy, many rationalists express a belief in god.
Later you might do Political philisophy (which has minimal religious stuff, so no sin points there!)
If you're smart and enjoy thinking (plus you actually belief in God) I'd recommend philosophy. Clearly you're concerned about committing sin but it sounds like you're pretty relaxed about that anyway so the topic you study shouldn't make a difference, unless it was like an A level in atheism or how to be a satanist.
Remember the academic method is all about theories and realising there might not always be one right or wrong answer (at lesst in Humanities "
*EDIT* only just seen your PS :/