God made promises both to Abraham and to his descendant. God did not say, “and to your descendants.” That would mean many people. But God said, “and to your descendant.” That means only one person; that person is Christ.
Our fathers on earth disciplined us for a short time in the way they thought was best. But God disciplines us to help us, so we can become holy as he is.
For this reason Jesus had to be made like his brothers and sisters in every way so he could be their merciful and faithful high priest in service to God. Then Jesus could die in their place to take away their sins.