Gal 3:15-18 + 19-22

Gal 3:15-18
Paul offers a good example for all teachers/preachers. Take an example from a situation known to those you are teaching to illustrate the point you are hoping to make clear. Clarity is vital in teaching and good examples which answer the question, 'What is this like?' help make things clear. Let's see how Paul's example works.

v. 15 - self-evidently true, the point of entering a covenant, or contract, is that once it is signed it cannot be set aside or amended.
v. 16 - begin at the beginning, the promise comes first. To some Paul's point may seem like nit-picking, but these things are important. It would have been just as easy for God to make his promise to Abraham and his offsprings, as to make the promise to Abraham and his offspring. God doesn't make mistakes, what he says is what he means. Notice Paul says the offspring is Christ. In this context Christ, or Messiah, functions as a technical term that would have been recognised from the OT as designating the promised King in God's Kingdom. In itself this is not a point that would have been challenged by first Century Jews. To identify the Christ as Jesus of Nazareth would have been, and still is for many.
Paul's point here, however, is not to identify the Christ, but to clarify that the promises were made to Abraham and to the Christ.
v. 17 - the law, given at Mt Sinai, was added after the promise. On the principle of v. 15 the law added later does not set aside or amend the promise.
v. 18 - draws an obvious conclusion. The promise was of an inheritance, the law is not about this and cannot be about this.

Paul here shows us that the promises of God are made to, or about the Christ and through the promise, the Christ an inheritance is given to Abraham and his children. The law, whatever its function, is not involved here.
What difference does it make for us that God gives us 'the inheritance' by promise and not by law? How should we live while we await the receipt of 'the inheritance'?

Gal 3:19-22
I'm sure you're asking, 'What then is the point of the law?' Paul anticipates this question and presents here one of the answers.
I think v. 19 means that because the law exists we know more clearly what transgressions are. Transgressions are breaking the law, crossing the boundary, before the law is promulgated we don't know where the boundary is. But once the law is declared when we break it we are rightly condemned as law breakers.
In this the law functions to restrain wickedness. We now know within which boundaries good living, right living occurs. If we remain within the boundaries of the law then wickedness is held in check, which is a good thing for all society.
In vv. 21-22 Paul addresses another objection. The law and the promises are not set against one another. The law was never able to give life (the inheritance) to humanity, because of our human weakness and inability not to sin (break the law). The law serves good purposes in the plans of God, one of which is to restrain wickedness, the promise offers life through faith in the Christ.