Galatians 2:18 For if I build again the things which I destroyed, I make myself a transgressor.
19 For I through the law am dead to the law, that I might live unto God.
Paul is simply saying here if he goes back to following the law and saying this is the way to salvation then he is becoming a a transgressor of the true word of God. This is just saying if you go back into sin after you have received forgiveness for the sin, you make yourself a transgressor. The next verse tells s that through the we are dead to the law because it it is of no consequence to us any more for we are born again unto Christ. We are a new creature no longer under the law but under grace. Jesus became our substitute for our sin on the cross. He fulfilled the law for each of us who will accept it. The law is of none effect in the believer. The price has been paid in full.Sin for the Christian, died on the Lord’s body on the cross. We live in Him.