Commentaries:
The promises to Abraham include that he would be heir of the world. Jesus Christ confirmed those promises and became Heir of them. "And if you are Christ's, then you are Abraham's seed, and heirs according to the promise" (Galatians 3:29)! From the time of Abraham, God has been working to establish, preserve, and expand Abraham's family and fulfill His purpose.
John W. Ritenbaugh
God's Promises Are Sure!
God justifies us. He does it freely. One of His purposes for doing so is that the promises may be sure.
The Jews have a saying, "How can a man enter into a right relationship with God so that he, too, may inherit the promises?" They understood the promises were made to Abraham and to his seed. They wanted to be able to participate in it, so they posed this question. Their answer: "He must do so by acquiring merit in the sight of God through doing good works, which the law prescribes." That is to say, by one's own effort.
However, as Paul describes here, justification through works, if it were even possible, would destroy the promises of God because no man can keep the law fully! If nobody can keep the law, because the giving of the promises depends on keeping the law, then God cannot give the promises. No one would ever qualify.
So God, wanting to ensure that the promises are given, justifies a person of His own free will. He blesses us, so that we can qualify to receive the promises on the basis of His grace. We had better be glad He does it this way, or we could never be co-inheritors of what Abraham is promised.
John W. Ritenbaugh
Grace Upon Grace
Paul states a major reason why justification absolutely must be by grace through faith: It is by means of grace that everybody receives a fair chance for salvation. Grace levels the playing field. What would happen to those who could not match the impressive body of works of an Abraham? They would fail to be justified. The truth is that even Abraham was not justified by his circumcision works. He, too, was justified through the sacrifice of Jesus Christ and God's grace. It has been this way from the beginning; Genesis 6:8 testifies that Noah found grace in God's eyes.
John W. Ritenbaugh
The Christian Fight (Part Four)