The difference is in the doctrine. The core doctrine of Islam is the Quran, which is seen as the direct word of God and the scripture contained within the Quran is view by all Muslims to be infallible, immutable and perfected. The core doctrine of Christianity is the four gospels (the only canonical books to contain the words and teachings attributed to Jesus Christ). The difference is that the Quran and the earliest Hadiths (commentaries) forms the basis for a legal system (i.e. Sharia law) Sharia law, in varying forms, is the law of the land in all Muslim-controlled countries (Sharia is also being practiced by parallel courts within the UK) while Christianity does not come with a legal system baked into it's text.