http://orthodoxinfo.com/general/stjohn_islam.aspx

Although not from a Catholic site, this was an interesting read, about what a Doctor of the Church had to say on the doctrine of Mohammed, who had lived shortly before his own time.

He called it not a new religion, but a heresy. I remember old writers like Belloc styled it in the same way. He gives brief answers about the veneration of the Cross, and the deity of Christ, reasoning with them from their own book. The Saint mocks the character of Mohammed, and the general disorganized structure of the Quran. He presents them with reasons to believe to Christ, and tells them they were misguided in accepting their prophet without divine signs of his credibility.