The passage
      v15. Using a legal analogy, Paul makes the point that even a human agreement (eg. a will, or deed), legally ratified, is irrevocable. So obviously, when God makes an agreement, it is set in concrete.
      v16. In the agreement God made with Abraham (obviously an irrevocable agreement), he promised blessings to Abraham's "seed". He promised a land, people as the stars in the sky, extending to the whole world (Gentiles); he promised the free gift of a kingdom. The fulfillment of this promise is in Abraham's "seed". Paul makes the point that although "seed" would naturally be taken as plural, it can rightly be taken as singular, and given the realization of the promise in Christ, it is obviously singular. Jesus is the heir of the promise and he determines the fellow-heirs.
      v17. Given the human analogy of v15, it is obvious that God is not going to annul the agreement made with Abraham. He is certainly not going to annul this promise just because he gave the law 430 years later.
      v18. The blessings of the kingdom are promised as a free gift. If they were by law-obedience then they would not come my means of God's promise, yet we know that God promised the blessings of the kingdom to Abraham as a gracious gift. The conclusion is obvious; the inheritance is not appropriated and maintained by law-obedience.
      v19. If the law does not serve to secure the inheritance (the blessings of the kingdom), what purpose does it serve? In v19b-22, Paul sets out to answer this question showing the inferior nature of the law:
        [i] The law was given to expose the true nature of sin and thus demonstrate to the sinner their state of loss before God. Thus, its ultimate purpose was to "lead us to Christ that we might be justified by faith", v24, ie. the law tells us we need a saviour. "Because of transgressions" is a poor translation. ie. it implies cause rather than purpose. The phrase can carry other meanings. eg. The law was added to (a) restrain evil, (b) provoke sin. Both suggestions are probably invalid.
        [ii] The law was given to deal with a specific situation, namely, a temporary interim dispensation until the promise was fulfilled in Christ.
        [iii] The law was given, not directly by God, but by His angels through Moses, and therefore cannot carry the same weight as a direct promise from God.
      v20. [iv] The law was conditional, and therefore limited, because angels (plural) gave it through Moses to people (plural).
      v21. Although the law is inferior to the promise, it does not work to contradict the promise. Paul demonstrates this truth by again defining the function of the law, a function quite separate to the promise. First, in v21b, he states the function negatively. It was not given to "impart life." The law was not given to promote righteousness such that a person may maintain their standing before God through obedience to the law.
      v22. Second, Paul states the positive function of the law. He does this by using the image of a jail sentence. The scriptures, containing the law, are like a magistrate passing sentence on a guilty person, and sin is like the jailer who shuts us up in prison. So the law serves to establish our condition of loss and therefore drives us to seek the promise through God's already revealed method. The promise of the kingdom (for Paul this means the gift of right standing in God's sight [justification], and the gift of the Holy Spirit) belongs to Christ (he is the inheritor of the promise made to Abraham), and this promise is given to us "through faith in Jesus Christ." So, the kingdom is for those who believe, and not those who "do".

The purpose of the law
      There is no contradiction between law and grace. Law serves the business of grace. The "true effect of law is to nail man to his sin. As the prison holds the prisoner, so man is shut up by the Law under sin. Rightly understood then, the Law prevents any attempt on man's part to secure righteousness before God in any other way than that promised to Abraham", A. Schlatter.
      Although the law can serve as a guide to the Christian life, its prime function is to expose sin and thus, our condition of loss, and in so doing drive us to rest on God's promise of grace now resident in Christ. Thus, the blessings of right standing in the sight of God and the gift of the Holy Spirit, are ours as by grace, appropriated through faith in Christ and not of works. To return to law to affirm our right standing, assure God's good pleasure, or progress our Christian life, is to renew our condition of loss. The scriptures continually remind us that our Christian walk must be by faith and not faithfulness (godliness).