God promised the land of Israel to Abraham and his seed.
And the LORD appeared unto Abram, and said, Unto thy seed will I give this land: and there builded he an altar unto the LORD, who appeared unto him. Genesis 12:7
The fact is, Abraham had two sons. One was a son through a slave. One was a son through his wife, Sarah. The son of the promise was the son through his wife. That son’s name was Isaac.
Fast forward through the ages and there was the seed of Abraham who was promised. The life of Abraham and Isaac contains a type and shadow of Jesus. Suffice it to say, Jesus was the seed promised of Abraham.
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. Galatians 3:16
Very early, God had Jesus in mind. He had him in mind when he talked to Adam and Eve that a woman would come whose seed would crush the head of the serpent. Again, God had Jesus in mind when he talked to Abraham about a seed. Jesus became the seed that God the Father sowed to save the world.