What is the relation between Christianity and Islam There are some common points that exist between Christianity and Islam. 
      What are the similarities and disagreement between Christianity and Islam?
The fundamental claim of Islam is that there is only one God and Mohammad is his prophet. This is believed because it is written in the Qur'an. The Qur'an is believed to be true based on Mohammad's testimony that he received it from the angel Gabriel.

Mohammad claimed to be the last in a long line of prophets, including most of the major Old Testament figures like Adam, Noah, Abraham, Moses, David, Elijah, etc., as well as Jesus. He claimed that the messages God gave to the other prophets were true (and were one with Islam). Muslims have concluded that the real Jesus and his message have since been lost or corrupted and what we now know to be Christianity is not reliable (because it disagrees with the Qur'an). Islam asserts that it is in accord with the real teachings of the prophet Jesus (which we no longer have in a reliable form). The Qur'an asserts that Jesus was not divine and never died (and, by implication, did not come back to life). 

If you don't wholly accept its claim that Islam has been around since the first prophet, Adam, then Islam is post-Christian. Mohammad appears to have known a little about Christianity (and perhaps Judaism). What he wrote down shows some familiarity with Christianity but also a severe misunderstanding of it and complete ignorance of some historical details (e.g. placing Samaritans with Moses).

Its relationship to Christianity is that it attempts to supersede Christianity by incorporating selected elements of Judao-Christian beliefs while simultaneously denying the fundamental beliefs both of Judaism and Christianity. Islam is logically incompatible with both Christianity and Judaism as we understand them. Judaism points to Jesus. Nothing in Christianity points to Mohammad. The movement started by Mohammad is superficially rooted in Judao-Christian history, but differs in important facts all along the way. As a movement, it is completely distinct and separate from Christianity, and its fundamental beliefs are mutually exclusive with those of Judaism or Christianity.