S1: Some people doubt that God exists. S2: Belief in the existence of God is universal. The second sentence:

The first sentence states that some people doubt God's existence (implying belief is not universal). The second sentence claims belief in God is universal (implying everyone believes). These two assertions are logically incompatible - if some doubt, belief cannot be universal. This is a direct contradiction. Therefore, the second sentence contradicts the assertion of the first.