Maimonides states that the popularity of Christianity (and Islam) is part of G-d's plan to spread the ideals of Torah throughout the world. This moves society closer to a perfected state of morality and toward a greater understanding of G-d. All this is in preparation for the Messianic age.
Or is the Islam part added?
I can think of one part of G-d's plan that Islam fulfilled. When Omar Ibn Al Khattab conquered Israel from the Byzantine Christians, the Byzantines had instituted a policy of conversion or death for the Jews of Israel. If Omar had not conquered the Byzantines, these Jews would've been wiped out.
Also Omar allowed Jews to live and pray in Jerusalem. The Romans had a policy that Aeolia(Jerusalem) was off limits for Jews, Constantine and later Christian monarchs would allow Jews only to come to Jerusalem on Tisha B'Av to mourn and cry over the loss of the Temple.
I know it's hard for you to realize this because you are not genocidal anti-semite. But the vast majority of Christians were and were it not for the Christians being busy fighting Moslems, I'm pretty sure the Christians would've wiped us out completely.
I know that you and most Christians that you know are decent people who don't wish to annihilate the nation of Israel from the face of the earth. Please don't be so naive as to think that you and your friends are indicative of the way Christians have been throughout the ages.