The case is clear in the case of Mohammed. He seduced the people by promises of carnal pleasure to which the concupiscence of flesh goads us. His teaching also contained precepts that were in conformity with his promises, and gave free reign to carnal pleasures. In all this, as is not unexpected, he was obeyed by carnal men. As for proof of the truth of his doctrine, he brought forward only such as could be grasped by the natural ability of anyone with a modest wisdom.
Indeed, the truths that he taught he mingled with many fables and with doctrines of the greatest falsity. He did not bring forth any signs produced in a supernatural way, which alone fittingly gives witness to divine inspiration; for a visible action that can only be divine reveals an invisibly inspired teacher of truth.
On the contrary, Mohammed said that he was sent in the power of his arms--which are signs not lacking even to robbers and tyrants. What is more, no wise men, men trained in things divine and human, believed in him from the beginning.
Those who believe in him were brutal men and desert wanderers, utterly ignorant of divine teaching through whose numbers Mohammed forced others to become his followers by the violence of his arms. Nor do divine pronouncements on the part of preceding prophets offer him any witness.
On the contrary, he perverts almost all the testimonies of the Old and New Testaments by making them into fabrications of his own, as can be seen by anyone who examines his law. It was, therefore, a shrewd decision on his part to forbid his followers to read the Old and New Testaments, lest these books convict him of falsity. It is thus clear that those who place any faith in his words believe foolishly.
Thomas Aquinas, Summa Contra Gentiles, Book 1, Chapter 16, Art. 4.