Death for apostasy has been one of the laws of Islam ever since its early centuries. The basis for this law was a comment made by Muhammad during the conflict between Mecca and Medina. In the seesaw of this conflict some Meccans professed Islamic faith when it seemed Medina was winning, but then renounced that faith when it seemed Mecca was winning. It was about these hypocrites that Muhammad said, “Kill them.”
The word of Muhammad in this special context was the source of the law prescribing death for any Muslim who converted in an altogether different context, and however sincerely, to another faith. It has been a powerful deterrent to many Muslims over the centuries who have been tempted to convert to Christianity.
Even apart from this law Muslims are generally not inclined to convert to Christianity because of Islam’s basic teaching that God’s revelation to and through Muhammad supersedes God’s revelation in and through Jesus the Messiah, because Christians allegedly corrupted that revelation. For this reason just about all Muslims see conversion to Christianity as a return to an earlier and inferior religion, somewhat similar to the way most Christians would see conversion to Judaism as a return to an earlier and inferior faith.
In past centuries few Muslims have therefore converted to Christianity. Much of this may be ascribed to Islam’s imperial power at the time Christian Europe was in the Dark Ages. But since the modern rise of the predominantly Christian West, Christians have engaged in missions to Islam, and many Muslims have been exposed to a new look at Christianity. A few have converted to Christianity but not been killed. Still, for most Muslims, Islamic law still stands as authoritative in the back of their minds, much like the teaching of Jesus stands in the back of the minds of Christians, even where they have not followed this teaching in actual life.