If a baby doesn't have the capability to be a theist, then they're necessarily an atheist.I hesitate to call a baby born in a certain country a member of that country. In a hypothetical situation, a baby born in America could be taken to England, raised British, be considered British, and remain oblivious to American heritage. A baby really can't even tell you their gender identity or sexual orientation, they don't know that something outside of their vision exist for awhile, they don't know of their own existence for awhile. How can it be said they are either when they don't even realize they exist?
From the earliest age we can measure it, it seems that babies tend to attribute things to agency in a way that's akin to a rudimentary animism, but falls short of theism. I feel safe in assuming that in the time between birth and these measurements, babies don't develop full-blown theism and then regress into something less detailed and sophisticated by the time we can measure it.