The Big Picture for this section is that God made promises, which would be received by faith, and those promises can never be changed; then He gave the Law–which serves a different purpose, it does not justify, cleanse, or grant righteousness. It does not draw us near to God. Abraham was made righteous by faith in God, so are we; he looked forward to God fulfilling the promises, and we recognize Christ as the fulfillment of the promises. Christ came, the seed to Whom the promise was made–The God Man, the perfect Mediator. He can impart life, the law cannot.
In verse 15 Paul asks his audience to consider the customs of their day–wherein if a man makes a covenant, a will–it would become legal and binding upon registering with the local office. It could never be changed, not even by the original writer. The inheritance was now guaranteed to whom it had been written of. Likewise, the promise is guaranteed to those to whom it is made, that we would receive the inheritance by faith–not by law-keeping or any other self effort.
Logically then, moving from the lesser to the greater, if man’s promises are legally binding, and inheritances entered according to law cannot be changed, then surely God’s promises can be trusted, they are unchanging. God will not void the promise, nor can we.
The promises were spoken to Abraham and his seed–Christ. Read through Psalm 2 for a glimpse of this promise making between Father and Son. Before the world was created, the Trinity was One in purpose and in delight, concerning this promise.
In verse 17-18 Paul sums up what he is teaching concerning the Law, that it was added 430 years later and does not change the covenant promises. The inheritance we are promised is guaranteed by God’s work, not ours. The Law does not change the promise, add conditions to it, tweak it, or void it. If God has declared we shall be justified by faith alone, the Law given by God does not change that.
We are being asked to focus on our inheritance, on the promises. We would do well to pause and ponder these. Hebrews 11:8-16 reminds us of our true inheritance begin a heavenly country (not an earthly one; no ‘Christian’ country is ever intended by God, America is not ‘blessed’ by God as part of any covenant promise. This world is passing away, and will be done away with in the end; our hope is in heaven, our inheritance there is eternal and everlasting):
By faith Abraham, when he was called, obeyed by going out to a place which he was to receive for an inheritance; and he went out not knowing where he was going. By faith he lived as an alien in the land of promise, as in a foreign land, dwelling in tents with Isaac and Jacob, fellow heirs of the same promise; for he was looking for the city which has foundations, whose architect and builder is God…All these died in faith, without receiving the promises, but having seen them and having welcomed them from a distance, and having confessed that they were strangers and exiles on earth. For those who say such things make it clear that they are seeking a country of their own…a better country, that is, a heavenly one. Therefore God is not ashamed to be called their God; for He has prepared a city for them.
“Why the Law then?” Verse 19-22 remind us that God’s Law had an intended purpose, though not to justify us. Paul here compares the Law to a Prison Warden. It keeps us shut up in our sin until the Mediator would arrive “so that the promise by faith in Jesus Christ might be given to those who believe.” The Mediator would be of both parties, fully God and fully man. In Him the fullness of deity dwelt, and He was born in the same flesh of ours, fully human. The Law was added for a seperate purpose, to point out our transgressions, to make us aware of our sinfulness, to show us our great need for a Mediator. How else could we approach a holy God seeking forgiveness? Not through Law and perfect law-keeping, but through Christ our Mediator.