Technically, atheism means that you do not believe in God. A baby has no concept of God, and so does not believe in It. Atheism is not necessarily a rejection of it.
For example, I'm going to guess that you have never heard of the god Tezcatlipoca. You have not rejected a belief in this god since you have never heard of Him. But you still don't believe in Him. You are an aTezcatlipocan.