I believe lots of theists believe in God without believing in the possibility of an empirical truth. I've certainly met Christian scientists (not to be confused with Christian Scientists) who fall into this category. It would be silly to call them agnostics.

As another poster wrote, often times agnostics approach the question as one of knowledge, whereas most theists as well as "weak" atheists see it as a question of belief. We all (well, almost all) believe all sorts of things that we do not truly know.

Agnostic theism