There’s something that’s wrong with saying atheism is just a lack of belief in God. The problem is that one can have a confidence level of 51% that God exists. But would we say that they believe that God exists? No.
If I have a confidence level of 51% that it is going to rain tomorrow, that is obviously not enough for belief. While I can’t be sure exactly what confidence level warrants outright belief, it is almost surely not 51%. A lot of epistemologists say belief comes into play in the “60’s”.
So, the person in our case has a 51% confidence level that God exists, but they lack belief in God. But would we call them atheist? That doesn’t seem right. It seems that we would call them agnostic because they neither believe nor disbelieve in God. The agnostic is on the fence in the same way someone is on the fence about whether or not the number of stars is odd or even. Hence, this type of agnosticism, call it agnosticism simpliciter, cannot be combined with theism or atheism.