There are three main religions (with many variations and sects from there) that all start with the same foundation. Christianity, Islam and Judaism all recognize the first 16 chapters of Genesis with relative equality. The first significant split is where Islam splits from the other two in Genesis 17:21. Let’s set the scene leading to this pivotal verse. God has promised Abram to be the Father of Many Nations. This promise was so late in his life that he and his wife Sarai actually laughed at the thought, for they were both old and Sarai was barren. After enough time had passed, Sarai tells her husband to sleep with her maid Hagar so he could have the son God had promised. So technically, Abram’s first son is in fact Ishmael. However this is NOT the first son of the couple, just of Abram.
So another decade passes and God comes to Abram again and this time, He’s a little more clear about his covenant. First he renames them to Abraham and Sarah and says in verse 16 “I will bless her, and moreover, I will give you a son by her. I will bless her, and she shall become nations; kings of peoples shall come from her.” This new 2nd son (first to both Abraham and Sarah) creates contention between Sarah and Hagar and who is the “rightful heir” so to speak. So God makes this perfectly clear in verses 20-21 “As for Ishmael, I have heard you; behold, I have blessed him and will make him fruitful and multiply him greatly. He shall father twelve princes, and I will make him into a great nation. But I will establish my covenant with Isaac, whom Sarah shall bear to you at this time next year.”
The Islamic people grab hold of verse 20 as proof that Ishmael will be blessed with the twelve sons and father a great nation. But they miss the following verse. Although God blessed Ishmael, he did NOT continue the covenant with him. Instead, He specifically says that the covenant will be with Isaac. That division thousands of years ago is the primary basis for the spiritual battle between the Islamic Nations and the Judeao/Christian nations. The Muslims still hold on to the fact that Ishmael is the father and therefore their Arab nations are the “Great Nation” they expected.
The Jews however continued to follow the line of Isaac as God had promised in v. 21. From that point until the coming of Christ, there were only two significant religions that both believed in the One True God. Then later throughout the struggles of the Israelites, the prophets continued to point to a coming Messiah that would eventually create a new covenant with God chosen people. The entirety of the Gospels is full of this new division and that will be saved for another day.