And this I say, [that] the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
Note 23 at Ga 3:17: Here, Paul was drawing a conclusion from the argument he began in Ga 3:15. If a man's covenant cannot be added to or taken away from, then God's covenant of promise to save people through faith in Christ could not have been affected through the introduction of the Law 430 years after God's promise.
The Living Bible paraphrases verses 17-18 in the following way: "Here's what I am trying to say: God's promise to save through faith--and God wrote this promise down and signed it--could not be canceled or changed four hundred and thirty years later when God gave the Ten Commandments. If obeying those laws could save us, then it is obvious that this would be a different way of gaining God's favor than Abraham's way, for he simply accepted God's promise."