“…knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified.”
In this verse, we have an affirmation and a denial. We have an affirmation of how God declares a person justified – He justifies him by faith in Christ’s death to forgive sin. We also have a denial: God does not justify a person by “the works of the law.”
Paul is in the process of dressing down Peter for his reversion into legalism (Ga 2:11-15). Upon the arrival of legalists from Jerusalem, the situation projected Peter into a predicament. He left fellowship with Gentiles and relapsed back into legalism with the Judaizers. He tried to accommodate both law and grace at the same time. That never works.
The tense of the word “knowing” shows that Paul and Peter knew that the law could not justify a sinner or sanctify a saint at a point in the past in the past and that they continued to know it (perfect tense). Peter failed to apply what he knew about the principle of grace. Paul appeals to common knowledge about the principle of grace.
Christians and non-Christians must grasp the principle of grace very clearly.
Do you have a clear understanding of justification by grace alone and faith alone? No matter how cultured, religious, well-behaved, gracious, philanthropic, or well-educated we might be, if we do not accept that only faith in the death of Jesus Christ forgives sin, God will not justify us.
We may have a good standing with people, but we have no place with God. A good reputation in our community will not impress God when we meet Him in Heaven. That will get us a big funeral but not eternal life. Good standing with God comes by belief in the death of Jesus for our sins.