Acts 7:8

And He gave him the covenant of circumcision; and so Abraham begat Isaac, and circumcised him the eighth day; and Isaac begat Jacob; and Jacob begat the twelve patriarchs.

For Abraham the promises of God provided one trial of faith after the other. Long before he had a son, the Lord told him that his descendants would be enslaved in a strange land, where they would be kept a matter of some four hundred years, Genesis 15:13, the exact number being given in other passages of Scriptures, Galatians 3:17. Incidentally, however, there was comfort for Abraham in the fact that God promised to judge, to speak the condemning sentence upon, the cruel masters, in order to bring His people out eventually to serve, to worship Him in this place, in Jerusalem, Exodus 3:12. Still later God gave to Abraham the covenant and the rite of circumcision, as the first sacrament of the Old Testament Church, and when finally Isaac was born, he was received into the covenant by this rite. And so, in due course, Jacob was begotten, and finally the twelve patriarchs, the sons of Israel.