In my library travels, I found a book in French published in Lebanon called "True Monotheism". It was of course a Muslim book, and its argument was essentially that Islam is the truly monotheistic religion, since it insists on the unity of Allah and that only he should be given total glory. Having been very bored recently, I have also been perusing many Protestant sites, and they seem to be saying the same thing vis. Catholicism. However, the latter are attacking such things as the cult of the saints, the God-given authority of ecclesiatics, etc. For some reason, in skimming this book, the Protestant argument in its most radical forms seemed to be metamorphizing in my head into the Islamization of the Gospel.
St Maximus Confessor in his commentary on the Lord's Prayer wrote that it is in the Trinity that the correct conception of God as one and many is truly manifested. And only in a correct Trinitarian concept of the Church and the cosmos is the Gospel truly understood. God's honor and glory in the Church of the Gospel is diffuse and shared in some very crucial ways. This is not polytheism or idolotry, though it can degenerate into something decadent as all things can. This Trinitarian conception, the understanding of the one and the many, is perhaps the ultimate tradition that even many Catholics have forgotten. It is the understanding that God does not want to reign alone in the universe, and His splendor and power consume all things.
Perhaps without this understanding, a Calvin and a Muhammad might be saying many of the same things.