Genesis 17:19 God said, “No, but Sarah your wife shall bear you a son, and you shall call his name Isaac.[a] I will establish my covenant with him as an everlasting covenant for his offspring after him
Who was Isaac
The Messiah was to be a descendant of Isaac. Isaac was the son of Abraham and Sarah. Sarah had not bore any children until a very old age. God appeared to Abraham and told him she would birth a child when Sarah was about 90 years old.
Abraham was 99 years old and had a son, Ishmael, through Hagar, the Egyptian slave. But God’s covenant promise would not be through Ishmael, but through the child, Sarah would bore. Isaac would be in the line of blessings God promised Abraham he would fulfill.
Abraham had two sons, Isaac and Ishmael. God promised Isaac that the Messiah would be through his family line.
Genesis 21:12 But God said to Abraham, Be not displeased because of the boy and because of your slave woman. Whatever Sarah says to you, do as she tells you, for through Isaac shall your offspring be named.
The prophecy fulfilled
The gospel writer Luke in the New Testament details for us the baptism and genealogy of Jesus.
Luke 3:34 the son of Jacob, the son of Isaac, the son of Abraham, the son of Terah, the son of Naho
Luke works the genealogy in reverse order, Matthew uses a chronological list. In this gospel Luke starts with Jesus and works it all the way back to Adam, the first man. We find Isaac in the genealogy record of Jesus.
This way Luke shows us how Jesus is related to the whole human race. Luke also traces the bloodline of Joseph to David.
The bloodline of Jesus' family
Jesus fulfilled a number of prophecies that have to do with the Messiah's genealogy, or family line. God narrowed down the family line of the Messiah in such a way that eliminated most people who have ever been born.
John 5:39 You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me
Jesus told the religious leaders to search the Scriptures and discover where they spoke about Him. The coming Messiah was to be born in one particular family.
This is one of the details of the Messiah's family line that was fulfilled by Jesus
The coming of the Messiah was predicted in the Old Testament. Jesus fulfilled these prophecies and, in doing so, has the right to claim to be the promised Messiah.
The Messiah would be a descendant of Isaac