It is clear that, in using the phrase ‘we who are Jews’, Paul has been speaking of himself and Peter not simply as Jews, but as Jews who had believed in Christ. As such, they both knew something: they knew that a man cannot be justified by works of the law.
What is justification? Justification is a declaration of God that we are viewed as not guilty. It is the formal legal pronouncement of the court which is the opposite of condemnation. God is willing to impute Christ’s righteousness to us as if it was our own righteousness, and to view us as if we were those who had never sinned. He chooses to see us as righteous men and women even though he knows that we are not, and to see this as our legal status before him. This act of God is so sure and stable and unchallengeable that Satan can find no fault with it, though he tries with all his might to do so. Nor can God’s law find fault with it, for the way in which God justifies fully satisfies the law, both in the punishment it demands and in the righteousness which it requires for us to enter heaven.