Agnosticism is not a religious viewpoint. You can be an agnostic theist or an agnostic atheist. The atheistic viewpoint doesn't say God doesn't exist, it is just the lack of a belief in God due to the lack of evidence to justify believing in God. I don't know if God exists or not, but it is only logical to not accept the claim that indeed God does exist because there is no justification for it. Just like how there is no justification for Zeus, or Thor, or Horus, or Odin, or Allah, or Krishna etc.