Powell said

The Constitution protects religions... freedom to worship
any notion of God. Separation of church and state is
designed to protect the citizen from governmental hindrance
of religious expression, too.

The Constitution doesn't protects atheists, for example,
because it isn't a religion.

Here is what it actually says....

Congress shall make no law respecting an establishment of religion, or
prohibiting the free exercise thereof;

I would say the constitution protects an athiest's choice not to execise any
any religion. It also clearly states that the government will not favor any
religion over any other religion. I think one could also say that atheism is a
particular "notion" of god and by your own argument is also protected.