Dr. J. G. Vos writes:
“[For Muslims,] salvation is obtained by human works. It is characteristic of religious Muslims that they are proud or self-righteous to such a degree that it is extremely difficult to get the Christian gospel of sin and redemption across to them. …
“Why is Islam so successful? How can its rapid spread be explained? And why is it so hard to win Muslims for Christ? A Muslim student once asked the present writer why Islam is so much more successful than Christianity. After a moment’s thought the reply was given that Islam is an easier religion than Christianity to live up to; it makes less difficult moral demands upon people. There is nothing in Islam to lead a man to say, ‘O wretched man that I am, who shall deliver me from the body of this death?’ or ‘I know that in me, that is in my flesh, dwelleth no good thing.’ A religion with reasonable attainable objectives fosters self-confidence, complacency, and spiritual pride—it leads inevitably to self-righteousness, but it does not give the sinner the anguish of a guilty conscience nor the frustration of trying without success to attain in practical living the requirements of an absolute moral standard. In brief, Islam makes a man feel good, while Christianity necessarily first (and often thereafter) makes a man feel bad. The religion of the broken heart is Christianity, not Islam.”
J. G. Vos, A Christian Introduction to Religions of the World (Grand Rapids: Baker Book House, 1965), pp. 65, 66, 67.