Do you believe in God? If so, you're an atheist. Because you believe in a God, a singular God, a particular God.
If you believed in all possible Gods, then I guess you deserve to be called a genuine theist. But such is rarely, if ever, the case. Religious true believers cleave to one God while rejecting the Gods that other people believe in.
So they're atheists in regard to all Gods but one.
Given the thousands of different religions, this means that the difference between an atheist who rejects all Gods but one, and an atheist who rejects all Gods including that one is very small.
To me this is a great argument against God. It's Proof #28 of the thesis, God is Imaginary. Check it out on a website by that name. The notion is traced to a quote from Stephen F. Roberts:
"I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours."
When I was a devoted member of an Eastern faith whose teachings centered on meditation practice conducted under the guidance of a guru who was considered to be God in human form, I used to think that fundamentalist Christians were way deluded.
"How could they believe in such strange stuff," I'd wonder. Jesus being born of a virgin, dying because he took on our sins, coming to life again, returning in a Second Coming. All that is just so freaking weird!
Meanwhile, I was believing that at initiation the guru places an astral copy of himself in the disciple's body, brain, mind, soul, or somewhere (the exact location was vague); he can tell what any of his million or more disciples is doing at any given time if he wants to, and he appears at a disciple's death to guide the initiate to a proper place -- either reincarnation back on Earth or some heavenly realm.
Naturally I felt that what I believed made good sense, while those crazy Christians were off in la-la land.
Well, they had as much reason to reject my beliefs as I had to reject theirs. So we were all atheists regarding God except for the one particular form of God that was embraced for some indefensibly weird reasons.