We need to understand what Paul means by ‘while we seek to be justified by Christ’, or ‘seeking to be justified by Christ’ as the Greek says. What time is he referring to? Is this about the time we first came to Christ, that is, to our conversion? Or is it referring to time after that, when we began to live as believers who are justified by Christ? The answer to that will determine how the rest of the verse is interpreted, and verse 18 also.
All of us must let go of any pretence to righteousness outside of Christ and confess that without him we have only sin. We do not lose anything by doing this but we only gain. This is the confidence that a believer has. Of course to the outsider, to the Jew living in Paul’s day, it appeared that the Jew who turned to Christ had forfeited righteousness.
Who is Paul addressing here? Some think that he now switches back to the Galatians and stops addressing Peter, but there is no great change in Paul’s language at this point, and the arguments used are eminently suitable to correct Peter’s error.
The Jews do indeed have advantages: the advantage of having the Scriptures, of having God’s law, of God dealing with them as a nation, of having the prophets sent to them, but they think of their greatest advantage as something else. They mistakenly believe that they are inherently superior to the Gentiles, and that they are the favourites of God because they are descended from Abraham. Christ taught them that they were no different to any other fallen human beings.