How Does Islam Regard Christianity?

Muslims believe that their prophet Mohammed is the last of a long line of prophets, which includes many prophets found in the Bible, such as Moses and Jesus.  According to Islam, Allah revealed through these earlier prophets teachings that were substantially the same as found in the Koran, which was given by Allah through the angel Gabriel to Mohammed.  The teachings given to prophets such as Moses and Jesus, however, were later corrupted by their followers.  In other words, Allah revealed what Muslims call the "true" Torah to Moses, and later the "true" Gospel to Jesus, but Jews and Christians respectively distorted these into the Torah and Gospel as found in the present-day Bible.

This, of course, is but a brief summary of how Muslims regard Jews and Christians, and their two religions.  For a much more detailed explanation, listen to Robert Spencer, and read his website Jihad Watch.