You need theology because, lets face it, we all talk about God at some point in time. Wouldn’t you want to speak about God intelligently, with at least a basic idea of who, and what we are talking about? We are all theologians of sorts, why not be an informed one? Even atheists need some knowledge of theology or their arguments make no sense, (not that they ever do fully make sense). But then again, “only a fool says in his heart, ‘there is no God'”. On the other hand, if we have no knowledge of our theological tradition we could end up saying other things just as foolish. Other than the indispensible Bible, you could start with a very good book like Alister McGrath’s Introduction to Theology.