In his brief and (mostly) tightly argued book God, Freedom, and Evil, Alvin Plantinga writes:
Start with the “theology” part of natural theology. “Theology” means “the science of God,” in the Aristotelian sense of “science” -- a systematic, demonstrative body of knowledge of some subject matter in terms of its first principles. Of course, atheists deny that there is any science of God even in this Aristotelian sense, but for present purposes that is neither here nor there. The point is that a science is what theology traditionally claims to be, and certainly aims to be.