It is not clear exactly what is meant with this phrase. Presumably, if Paul were to teach that the old covenant laws are still in force, he would be violating God’s revealed will (Acts 10:1 – 11:30). This would be sinful and a denial of righteousness through faith in Christ.1 Alternatively, Paul has in mind the actual transgression that follows a re-establishment of the law as the authority in the believer’s life. If the law is once again held up as necessary for justification, then one must submit to all its demands and thus one is bound to become a transgressor of it (Galatians 5:2f.; Galatians 2:16).2 Essentially, if you return to the law you will find that you cannot keep it.3
18 For if I rebuild what I tore down, I prove myself to be a transgressor.