For even if many men once boasted that they worshipped the Supreme Majesty, the Maker of heaven and earth, yet because they had no Mediator it was not possible for them truly to taste God’s mercy, and thus be persuaded that he was their Father. Accordingly, because they did not hold Christ as their Head, they possessed only a fleeting knowledge of God. From this it also came about that they at last lapsed into crass and foul superstitions and betrayed their own ignorance. So today the Turks, although they proclaim at the top of their lungs that the Creator of heaven and earth is God, still, while repudiating Christ, substitute an idol in place of the true God. - John Calvin, Institutes of the Christian Religion (Book II, Chapter VI).
Calvin on the Christ-less Unitarianism of Islam