If a person believes in God, He is believing in Jesus with or without knowing it. If he believes that Jesus isn't God with the Father and the Holy Spirit, he is believing in an idol as though it were God. No two ways about it.
Take your pick. Either a believer in God is a Christian and knows it, or he is a Christian and doesn't realize that he is, or he is an idol worshiper but not a God worshiper.
So can someone believe in an idol as God, but not believe that idol is Jesus? Why not call that idol a God and just classify that person a a "God" worshiper?
One of the religious dichotomies that I've always questioned is how the monotheistic religions can reconcile many billions of believing in a different god than their own, and being as sure of that belief as themselves. I suppose with polytheism there's a bit more room for congruence than some other religions.
As a Christian, you would need to accept that the billions of Muslims are incorrect in their beliefs and worshipping a deity that's nonexistent, surely.