I hear Christians make this mistake all the time. They confuse what happened with Hagar and Ishmael as the separation between Judaism and Islam. It is not. Islam as a religion didn’t even come into existence until thousands of years after Ishmael was born. It is a religion that combines, plagiarizes, and borrows from Christianity, Judaism and Zoroastrianism. It is a religion that was born out of regional warring and politics. A truer statement would be to say it shares the same god with progressive liberalism. HA! The tactics of the two groups are not that fare apart, that is for sure.
That's exactly what happened. God said He would be a God to Isaac but never once said He would be a God to Ishmael. The Arabs are descendent of Ishmael. The Jews/Isrealites are descendents of Isaac. Abraham pleaded with God that Ishmael would live before God but God essentially said no.
Genesis 17:18 And Abraham said unto God, O that Ishmael might live before thee! 19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.
Christianity didn’t come into being for 2,000 years after Abraham, but he still believed in the Messiah and it did happen. Muslims are Ishmaelites and they still fight against the whole world and the world still fights against them. Isiah was born of freedom and Ishmael was born of slavery. Sin is slavery and belief in Jesus Christ is freedom from the slavery of sin.