Theism is the concept of a god. Theism is where theists and atheists come from. If you accept theism you are theist. If you don't accept theism you are an atheist. Babies are not atheists. Babies have no way to accept or reject the concept of a god. How can babies choose to be theist or atheist when they can't examine the evidence? Same goes for rocks, trees, cats and dogs. It has nothing to do with belief. Your belief or disbelief is the result of your choice, not the reason for it.
I am an atheist. After examining the evidence, I choose not to accept theism as a truth.