“And this I say, that the law, which was four hundred and thirty years later, cannot annul the covenant that was confirmed before by God in Christ, that it should make the promise of no effect”
And this I say,
Paul now explains verse fifteen [verse 16 was a parenthesis].
that the law, which was four hundred and thirty years later,
Moses received the law 430 years after the last ratification of the Abrahamic covenant, when Israel (Jacob) went to Egypt (Genesis 46:1-4); (Exodus 12:40). The law came 645 years after the initial promise to Abraham.
cannot annul the covenant that was confirmed before by God in Christ,
The words “confirmed before” means ratified before. God ratified His promise to Abraham on a number of occasions (to Abraham, Genesis 15; by vision, Genesis 15; by birth of Isaac, Genesis 21; by oath, Genesis 22). God confirmed this 430 years before He gave the law. He did this permanently (perfect tense).
that it should make the promise of no effect
God ratified the Abrahamic covenant before he gave the law to Moses, so the law could not annul the Abrahamic covenant. The coming of the law did not invalidate God’s promise of grace to Abraham because God gave him a unilateral promise [unconditional contract or covenant].
Law cannot abrogate grace.
God ratified grace permanently before He gave the law. The law was not an addition to grace. Once God signed the contract of grace, He would not go back on it.
Many people today throw out grace for law despite God’s promise to bless us no matter what. There are no conditions to His blessing, no strings attached.