Monotheists typically believe in one omnimax God. But how can they know that there are no other gods? Isn't it impossible to know for sure that no other gods exist? Wouldn't an honest monotheist admit the possibility of other gods?
And how can one possibly know that God is an omnimax God? It's impossible for any human to experience omniscience, omnipresence, or omnipotence, so how can they claim to perceive these qualities in God?
I would, rather, say that monotheists believe in the "only God"*; that would be why there's "no other Gods," because
God is only. It speaks to the image of "God" that the monotheistic person holds, ideally a monistic image. (The image of "God" is not God, but that's another story).
"Omnimax" refers to omniscience/omnipotence/omnipresence, which are one image. It can be seen as a metaphor for the image of "God" that is, in each moment, creator of all things, present in all things, and aware of all things.
* dictionaries reflect this