Muhammad enticed peoples with the promise of carnal pleasures, to the desire of which the concupiscence of the flesh instigates. He also delivered commandments in keeping with his promises, by giving the reins to carnal pleasure, wherein it is easy for carnal men to obey: and the lessons of truth which he inculcated were only such as can be easily known to any man of average wisdom by his natural powers: yea rather the truths which he taught were mingled by him with many fables and most false doctrines. Nor did he add any signs of supernatural agency… but he asserted that he was sent in the power of arms, a sign that is not lacking even to robbers and tyrants. Again, those who believed in him from the outset were not wise men practiced in things divine and human, but beastlike men who dwelt in the wilds, utterly ignorant of all divine teaching; and it was by a multitude of such men and the force of arms that he compelled others to submit to his law.
Lastly, no divine oracles of prophets in a previous age bore witness to him; rather did he corrupt almost all the teaching of the Old and New Testaments by a narrative replete with fables, as one may see by a perusal of his law. Hence by a cunning device, he did not commit the reading of the Old and New Testament books to his followers, lest he should thereby be convicted of falsehood. Thus it is evident that those who believe his words believe lightly.
* This excerpt is from “Summa Contra Gentiles” by St. Thomas Aquinas