The text to focus on is ST I, q. 3. Aquinas does not ask, in a generic and undifferentiated way, “Is God simple?” and then answer “Yes.” Instead, he develops a specific notion of divine simplicity that consists in the negation of certain specific kinds of composition. This is important, because if we assume that Aquinas holds that God is simple in just any sense we ourselves can imagine, then we will be likely to jump too quickly to the conclusion that he is contradicting himself when he says that Christ is composite.
