But, it is not as though the word of God had failed. For not all who are descended from Israel belong to Israel, and not all are children of Abraham because they are his descendants; but “Through Isaac shall your descendants be named.” This means that it is not the children of the flesh who are the children of God, but the children of the promise are reckoned as descendants. For this is what the promise said, “At the appointed time I (God) will return, and Sarah shall have a son.” (Romans 9:6-9)
God keeps all of his promises. Isaac was the promise, not Ishmael because God’s promise was for Sarah to have a son, not Hagar. Ishmael and his descendants are Muslim. Since gentile means not Jewish, Muslims are Gentiles also. As we discussed, if Gentiles want to be a child of Abraham we have to be adopted or grafted into the family by means of our belief in Jesus as our Savior. God gave Israel, the descendants of Abraham and Sarah, special privileges, and still they turned away from Him. God promising Sarah a child seemed impossible at her advanced age, so Abraham first asked God to fulfill his promises through Ishmael, and God refused because it was God’s plan for the Israelites to be the chosen people, not the Muslims. As Abraham took his journey with God as, hopefully, we are doing now, he learned to trust God and to know that God’s plan is perfect. Hopefully, we are learning to trust God in our own journeys as well. By the end of Abraham’s journey, Abraham was willing to sacrifice his own son as God requested of him. Of course, God did not require Abraham to sacrifice Isaac in the end, but he wanted to test Abraham’s faith perhaps to show the readers of the Bible, that we in our walk with God can also have complete trust in Him.
In Genesis we see in Abraham’s life: “there emerges a new nobility and independence, the outworking of Abraham’s developed habit of walking with God, resting in his revealed will, relying on him, waiting for him, bowing to his providence, obeying him even when he commands something odd and unconventional. From being a man of the world, Abraham becomes of man of God.”
Think of how Abraham must have felt when he thought that God was not keeping His promise to him. But God knew what was best for Abraham. I repeat, God transformed him from being a man of the world to being a man of God. God also wants to transform us from being people of the world to people of God. How quickly we learn depends on how wonderful our life will be. God will answer all of our prayers the best he can, but his ultimate concern is that we become His children, transforming from people of the world to people of God.
 “Knowing God,” by J.I. Packer, p. 93