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A contract is an exchange of services, whereas a covenant is an exchange of persons. And whereas a contract can be broken, a valid covenant is indissoluble. The solemn importance of covenants in the Pentateuch (first five books of the Bible) and beyond is also seen in God’s swearing sacred oaths in binding himself to his people. And whereas mere humans can and have strayed throughout salvation history, God will not. That is, God ensures that his covenants will be fulfilled, even when his people transgress seriously, such as through idolatry.
In Genesis 17:20-21, God tells Abraham that he will fulfill his covenant through his yet-to-be-born son Isaac, not Ishmael, whom Abraham had fathered through Hagar, his wife’s maidservant, in his desperate attempt to provide an heir to fulfill God’s covenants with him:
As for Ishmael, I have heard you; behold, I will bless him and make him fruitful and multiply him exceedingly. . . . But I will establish my covenant with Isaac, whom Sarah shall bear to you at this season next year.
God makes three covenants with Abram/Abraham regarding the latter’s descendants: 1) to make of them a great nation (Gen. 15), fulfilled in the founding of Israel by Moses (Ex. 24); 2) to make of them a great kingdom (Gen. 17), initially fulfilled in David and his royal descendants (1 Samuel 16ff.); and 3) to make of them a “catholic” or universal blessing (Gen. 22), i.e., one meant to impact all nations, and ultimately fulfilled in Jesus Christ, the King of kings, who restored and fulfilled the kingdom of Israel in founding the Catholic Church (Luke 1:26-33; Matt. 28:18-20).
God reaffirms that his covenants will be fulfilled through Isaac and his descendants (Gen. 21:12, 22:2, 14-18). God also makes clear that Hagar and Ishmael will be provided for otherwise (Gen. 21:13, 15-21).
For more on the importance of covenants in salvation history, please see our tract “Pillar of Fire, Pillar of Truth” and my apologetics essay “Pentecost and the Papacy.”