Our legalistic efforts to make ourselves righteous are contemptable to God because they are demeaning to the death of his Son, minimizing our sin. So Paul considered those filth at his conversion, and, additionally, as part of his ongoing Christian life, considers everything filth because of Christ. This means in comparison to Christ, not even the best things in life are in same ballpark, when choosing between any good thing in life and Christ, it‚Äôs Christ, and anything that hinders pursuit of Christ is out. Gaining Christ means being found in him, which means being so closely associated with him that God credits his righteousness to our account. This is done by faith.