Some will argue that if someone presents, for example, their “God” as teaching the rape of small children to be okay, then that God is not God at all. And that, I would argue, is true. It could be argued that that “God” would suffer from a moral defect, and therefore, could not be God.
But even if it is possible for a person (or a faith, like Islam) to claim belief in the one true God, but so distort what God teaches that he (or it) ceases to truly believe in the “one true God" in reality, then, according to the Church, Islam has not reached that point in its errors.