He does have a believing mind. Everyone does.
Unbelievers don't have believing minds, that's why they are called unbelievers silly.
He just doesn't believe in your god, just as you don't believe in the gods of countless other religions.
That's a problem, because God is the embodiment of truth and reality.
So, if you disbelieve in how and why the truth and reality is known to exist; being a unbeliever and all, when belief and a believer is required to know the truth and reality of God, then you are stuck in a delusion, because you disbelieve in the truth and reality of God.
But everyone, whether Christian, atheist, Jew, or Muslim, believes and knows plenty of things.
Not without being a believer can you know the truth and reality of God. And because it is only in and with a Believing Mind that the truth and reality can be known to exist and occur, then disbelieving the Owner of that Believing Mind exists and occurs eliminates you from reality. Sorry Bub, but no unbelievers allowed.
If you want to be a part of the truth and reality, then you are going to need to believe in Whom embodies the truth and reality to begin with.