Abraham, Moses and Christ (Galatians 3:15-18).
The legalistic false teachers based their doctrine on Moses, saying that our conformity to the Law of Moses was the way of salvation. Paul goes back 430 years earlier to Abraham, when God revealed the true way of salvation, based on promise (grace), not on law (works). He then uses the human example of a WILL, which, once established, promises and legally guarantees the inheritance to the seed, as a free-gift. He points out that once it is confirmed, it cannot be annulled, especially after the one who made the will has died. In the same way, God made an unchangeable covenant (will) with Abraham, promising that in him and his seed all the nations will be blessed (receive the inheritance of salvation), on the basis of the death of Christ, which was an established fact in the mind of God, from the foundation of the world. Therefore, this way of salvation by grace must still be in force, and that therefore the Covenant and Law of Moses did nothing to render this promise void. Thus, the Law was never meant to be a substitute way of salvation, replacing the Abrahamic promise, for if salvation was by the law, then it would no longer be through the promise (faith), and that is impossible, because God gave Abraham an unconditional, unchangeable promise of this inheritance (the blessing of Abraham), and God is always faithful to keep His word.