From a fine article 'Luther, Lutheranism, and the Challenge of Islam' by Adam Francisco in Concordia Theological Quarterly (Vol 71, 2007):
This motif that Islam is the aboriginal religion of humanity and history is prominent in the Quran. All the prophets beginning with Adam through Moses unto Jesus, Muslims allege, proclaimed essentially the same message that Muhammad preached. "God sent down to you (step by step), in truth, the Quran, confirming what went before it; and He sent down the Torah (of Moses) and the Gospel (of Jesus) before this, as a guide to mankind, and He sent down the criterion [the Quran]" (3:3, 9:111). Despite the obvious contradictions, however, Muhammad did not start a new religion, the Quran claims. Instead, he revived the religion of Moses and Jesus, messages had been corrupted (tahrif) by Jews and Christians who purposely altered the biblical text and skewed the message of Moses and esus. Thus, God sent Muhammad to reiterate what truth was left in the Judeo-Christian tradition and to secure the full revelation of God once and for all in the Quran.
In other words, Islam has a tidy way of explaining the historical fact of Christianity while not really allowing for it to have a seat at the table in terms of influencing the future development of Islam.
What do you think?