15 Friends, let me give you an example from everyday affairs of the free life I am talking about. Once a person's will has been ratified, no one else can annul it or add to it. 16 Now, the promises were made to Abraham and to his descendant. You will observe that Scripture, in the careful language of a legal document, does not say "to descendants," referring to everybody in general, but "to your descendant" (the noun, note, is singular), referring to Christ. 17 This is the way I interpret this: A will, earlier ratified by God, is not annulled by an addendum attached 430 years later, thereby negating the promise of the will. 18 No, this addendum, with its instructions and regulations, has nothing to do with the promised inheritance in the will.
Matthew Henry's Commentary on Galatians 3:15-18
Commentary on Galatians 3:15-18
(Read Galatians 3:15-18)
The covenant God made with Abraham, was not done away by the giving the law to Moses. The covenant was made with Abraham and his Seed. It is still in force; Christ abideth for ever in his person, and his spiritual seed, who are his by faith. By this we learn the difference between the promises of the law and those of the gospel. The promises of the law are made to the person of every man; the promises of the gospel are first made to Christ, then by him to those who are by faith ingrafted into Christ. Rightly to divide the word of truth, a great difference must be put between the promise and the law, as to the inward affections, and the whole practice of life. When the promise is mingled with the law, it is made nothing but the law. Let Christ be always before our eyes, as a sure argument for the defence of faith, against dependence on human righteousness.