This brings us to a very practical conclusion: God deals with us according to his promises, not according to our works. Everything God has to offer comes through a promise....Salvation in Christ does not rest on a law that we inevitably break; it rests on a promise that God cannot break. God has promised forgiveness of sins through the death and resurrection of Jesus Christ. He has promised eternal life to everyone who comes to Christ in faith. God will not—indeed, he cannot—go back on his promise. His covenant is an irrevocable will and testament. It stands firm forever."
From Philip Ryken's Galatians: Reformed Expository Commentary, 127-128.