Theism vs. Atheism vs. Agnosticism
Did you know that agnostics, atheists, and people born in Missouri have a patron saint? He's Thomas the Apostle. In his own words, "Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe." At that moment, was he a theist, an atheist, or an agnostic? Let's see.
Theism is generally defined as a belief in a god or gods.
Atheism is generally defined as a disbelief in a god or gods
Agnosticism is generally defined as not knowing if there is a god or gods.
The beliefs are different, but they have something in common -- not knowing. The existence of a god or gods cannot be objectively proven nor disproven, no more than the existence of flying saucers.
So, all three fit the definition of agnosticism. Frankly, I believe all humans are agnostics just as Thomas was, no matter how religious or non-religious they claim to be. They don't know (in the scientific sense) and can't know if there is a god or gods. They can only believe one way or another until proof is provided with objective evidence.