My "Sociology of American Religion" class had an excellent visit to our local Muslim school yesterday. The principal, Dr. Jitmoud, very helpfully explained the basics of Islam. He said something that I had heard before, but then gave it a further application I had not appreciated.
By the Muslim account, Adam is the first prophet of God (Allah), a witness to the Creator. In saying that Adam is the first prophet of God, Islamic thought thus reasons that Islam is the oldest religion. This also then makes sense of the claim that all people are really Muslims, most of whom need to be encouraged to return to the original religion. And this, in turn, helps account for why they are so hard on people who convert from Islam to other faiths.