Too long a story to go into, but there's evidence indicating an erased history that Christianity and Islam were a single religion at one time.

Even in the accepted (but suppressed) historical record and religious doctrine, there are incredibly strong links and associations between the two. A few examples:

• Muslims are awaiting the return of Jesus.
• The Virgin Mary, mother of Jesus, is stated to be the most holy woman in the Quran. Two of it's books are named for her.
• Muhammad's first wife was from a family of Christians, and was thus likely Christian herself.
• Muhammad directed Muslims--very strongly and in no uncertain terms--exactly how to treat all Christians in the Ashtiname of Muhammad. It's worth reading the entirety of the short document, but here's a taste:

Thus they [Christians] will live favored and protected from everything which may offend them by the Callers to religion (Islam), wherever they may be and in any place they may dwell.

Rather than study such things, from skimming the comments I see that people would rather give themselves over to ignorance, fear and hatred, rather than to the study of such things as I have mentioned. Sad.