Galatians 3:17 - Matthew Poole's English Annotations on the Holy Bible

The covenant, that was before confirmed of God in Christ: the word translated covenant, is the same as before; ordinarily signifying one's disposal of things in his last will and testament. Which name is given to the covenant of grace, with respect to the death of Christ; for though Christ as yet had not died, yet he was, by virtue of the covenant of redemption, and in God's counsels: The Lamb slain from the foundation of the world, Revelation 13:8. This (he saith) was in Christ, (as Abraham's promised seed), confirmed of God to Abraham, by God's oath, Hebrews 6:17,18; by frequent repetitions of it; by such solemn rites as covenants use to be confirmed by, Genesis 15:17,18; by the seals of circumcision, Genesis 17:11 Romans 4:11; by a long prescription, &c.; though it received indeed its final and ultimate consummation by the death of Christ, yet it was before many ways confirmed. The law, which was four hundred and thirty years after, cannot disannul: the law was given four hundred and thirty years after the giving this promise to Abraham: though, Genesis 15:13, the round number of four hundred years only be mentioned, which are to be counted from the birth of Isaac; yet, Exodus 12:40, they are reckoned (as here) four hundred and thirty years, from Abraham's going out of Canaan, Genesis 12:4; from whence to the birth of Isaac were twenty-five years, Genesis 21:5, compared with Genesis 12:4; from the birth of Isaac till Jacob was born, sixty years, Genesis 25:26; from thence till Jacob went down into Egypt, one hundred and thirty years, Genesis 47:9, where they abode two hundred and fifteen years. Hence the apostle concludes, that it was impossible that the law, which was not given till four hundred and thirty years after the confirmation of the promise, should make the promise confirmed of no effect.