Summary: In Galatians 3:15-18 four reasons are given for affirming the superiority of the covenant of promise: God presents its 1) Confirmation (Gal. 3:15), its 2) Christ-centeredness (Gal. 3:16), its 3) Chronology (Gal. 3:17), and its 4) Completeness (Gal. 3:18).
Many of the delightful things that we anticipate in our lives are based on promises. We enter into relationships with people we care about and in broader organizations through promises. The word “promise” itself is filled with hopeful realities. We love it when someone makes a promise. It has the intention of good will built into it. It is a kind of pledge of love, a pledge of loyalty, a pledge of faithfulness, and a pledge of integrity. It is a promise that good will come to us. Sometimes the word “promise” is called a word of honor. A synonym for “promise” would be “guarantee”, “vow.” or “bond.” Biblically, it would be called a “covenant.” (https://www.gty.org/library/sermons-library/48-16/gods-eternal-covenant-of-promise)
In Galatians 3, the Apostle Paul explains the nature of the covenantal promise of Justification. Having established that Scripture confirms the experience of the Galatians, namely, that God grants his Spirit to people because they surrender to Christ and not because they conform their lives to the Mosaic law, Paul now moves to another kind of argument, an analogy or “example from everyday life” (cf. also Rom. 3:5; 6:19; 1 Cor. 9:8). Through it he makes his point once again: the law of Moses is not God’s most important revelation; that revelation is God’s promise to Abraham. This means that the response demanded of Abraham is also more significant than the response demanded through Moses. That is, faith (Abraham’s response)—not works of the law—is the foundation of our relationship to God (cf. Rom. 4:13–15). (It is no wonder that Paul must soon cover his tracks and speak about the purpose of the law in Gal. 3:19–25.) (McKnight, S. (1995). Galatians (p. 165). Grand Rapids, MI: Zondervan Publishing House.)
How exactly is this is an issue relevant for us? In understanding the relationship between the promises of God and His law, what does God expect us to do in order to receive His promises? Which promises in Scripture are for us? Can circumstances change that cause God to change a promise?
In contrasting the covenants of promise and of law, Paul first shows the superiority of the one and then the inferiority of the other. In Galatians 3:15-18 four reasons are given for affirming the superiority of the covenant of promise: God presents its 1) Confirmation (GALATIANS 3:15), its 2) Christ-centeredness (GALATIANS 3:16), its 3) Chronology (GALATIANS 3:17), and its 4) Completeness (GALATIANS 3:18).
Justification of by faith is shown in the promises of God as seen through:
1) Its Confirmation (Galatians 3:15)
Galatians 3:15 To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. (ESV)
Galatians 3:15–18 is an analogy between human and divine covenants and more technically what we have here is a similitudo or simile. A simile is a bit different from an example (exemplum), though it has a force or rhetorical effect very similar to an example (Or. 5.11.22). Basically the force of the argument is strengthened to the degree that the things being compared are equal or nearly so. This form of proof is less powerful than for example the appeal to the Galatians’ experience, not least because it involves an artificial, or humanly devised proof. Paul has presented his two divine proofs, one from supernatural experience and one from the sacred Scriptures, and now he will turn to more mundane, merely human arguments, or as Chrysostom put it, Paul now uses human examples. Paul is not signalling the weakness of the analogy, only the humanness of the argument (Witherington III, B. (1998). Grace in Galatia : A Commentary on St. Paul's Letter to the Galatians (240). Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.
B.C. Before Christ)
There is a delightful change of tone here in Galatians 3:15. Paul now addressed the Galatians here as “brothers,” a term of endearment he had not used since 1:11, although it would occur again seven other times in the letter (4:12, 28, 31; 5:11, 13; 6:1, 18). Although the Galatians were confused, foolish, and bewitched, and although Paul felt betrayed, perplexed, and neglected from them, still they were adelphoi, “brothers.” This term of relationship is especially appropriate at the beginning of a passage that will seek to answer the questions: “What makes a family a family? Who are the true children of Abraham, the heirs of the promise, and thus entitled to call one another brothers and sisters?” (George, T. (2001, c1994). Vol. 30: Galatians (electronic ed.). Logos Library System; The New American Commentary (244). Nashville: Broadman & Holman Publishers).
First of all, the covenant of promise, the promise made to Abraham and his descendants, was superior because it was confirmed as irrevocable and unchangeable. This can be illustrated by reference to a human covenant. In terms of human relations, Paul says, even … a man-made covenant, … when it has been ratified, allows no one annuls it/sets it aside or adds conditions to it. Even human beings hold their covenants to be inviolable and unamendable. Once ratified, they are irrevocable and unchangeable. While the Judaizers might go so far as to agree with Paul that Abraham was justified by his faith, they would then add that the coming of the law changed the basis for gaining salvation. Paul here wanted to clarify that nothing would change the promise that God made to Abraham. Hence, no one annuls it/sets it aside or adds conditions to it. His point was not that God couldn’t change his covenant with us, but that God didn’t ever change the covenant established between himself and us, as illustrated by his specific dealings with Abraham. (Barton, B. B. (1994). Galatians. Life application Bible commentary (104). Wheaton, Ill.: Tyndale House.)