Some of the saints share their thoughts on Islam:
“Whoever does not embrace the Catholic Christian faith is lost, like your false prophet Muhammad.”
-St. Peter Mavimenus
“There is also the superstition of the Ishmaelites which to this day prevails and keeps people in error, being a forerunner of the Antichrist…. From that time to the present a false prophet named Mohammed has appeared in their midst. This man, after having chanced upon the Old and New Testaments and likewise, it seems, having conversed with an Arian monk, devised his own heresy. Then, having insinuated himself into the good graces of the people by a show of seeming piety, he gave out that a certain book had been sent down to him from heaven. He had set down some ridiculous compositions in this book of his and he gave it to them as an object of veneration.”
-St. John Damascene
“We profess Christ to be truly God and your prophet to be a precursor of the Antichrist and other profane doctrine.”
-Sts. Habenitus, Jeremiah, Peter, Sabinian, Walabonsus, and Wistremundus (d. 851), martyrs of Cordoba, Spain.
“Any cult which denies the divinity of Christ, does not profess the existence of the Holy Trinity, refutes baptism, defames Christians, and derogates the priesthood, we consider to be damned.”
-Sts. Aurelius, Felix, George, Liliosa, and Natalia (d. 852), martyrs of Cordoba, Spain.
“On the other hand, those who founded sects committed to erroneous doctrines proceeded in a way that is opposite to this, the point is clear in the case of Muhammad. He seduced the people by promises of carnal pleasure to which the concupiscence of the flesh goads us. His teaching also contained precepts that were in conformity with his promises, and he gave free rein to carnal pleasure. In all this, as is not unexpected, he was obeyed by carnal men. As for proofs of the truth of his doctrine, he brought forward only such as could be grasped by the natural ability of anyone with a very 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 be only divine reveals an invisibly inspired teacher of truth. On the contrary, Muhammad said that he was sent in the power of his arms—which are signs not lacking even to robbers and tyrants.”
-St. Thomas Aquinas