Isaac was not Abraham’s first born son. Abraham was 75 years old when God made the promise to him, “I will make of thee a great nation.” (Gen. 12:2) But his wife Sarai was barren. Ten years later, she was still barren, so she suggested to her husband that he also marry her hand maid Hagar that Sarai may obtain children by her. (Gen. 16:2) Hagar conceived and bore Ishmael, Abraham’s first-born son.
The Apostle Paul tells us in Gal. 4:22-26,28: “Abraham had two sons, the one by a bond maid, the other by a freewoman. But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. Which things are an allegory: [figurative illustration], for these are the two covenants; the one [Law covenant] from the mount Sinai, which gendereth to bondage, which is Agar. For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. But Jerusalem [Abrahamic covenant which produces the heavenly seed class] which is above is free, which is the mother [Sarah] of us all. … Now we, brethren, as Isaac was, are the children of promise.”
The Apostle Paul was speaking to the consecrated, disciples of Christ as we read in Gal. 3:27,29: “For as many of you as have been baptized into Christ have put on Christ. … And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise.” Ishmael was born before Isaac. Thus, it appeared that the Abrahamic promise would come through him, but God said no. “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.’”—Gen. 17:19
Thus, Sarah was a type of the Abrahamic Covenant and Isaac was a type of Jesus and the church. Hagar was a type of the Law Covenant and Ishmael was a type of Jewish people.