Then you are agnostic. An atheist makes the affirmative claim that God did not create the Universe

No, I'm an atheist, not an agnostic. I don't make an affirmative claim on why and how the universe exists, but I also affirm that it was not a God (any God) who created it.

We believe that God is eternal and has always existed.

Interesting. I will accept your point. If I believe that universe has an age, as science says, then it's reasonable for you to ask me what created it.

None of the thousands of other gods you are referring to claim to have created the Universe. They all exist within the Universe. Theism refers to the belief that a personal and knowable God created the Universe. The three theistic religions are Judaism, Christianity, and Islam.

Wrong. Theism is the belief in a God or Gods.

And other religions most certainly believe that their own God created the universe. For example under Hinduism it was the God named Brahma who created the universe.