I prove myself to be a lawbreaker: Abandoning faith and returning to the law places a person under the law's authority, where everyone faces God’s judgment (Romans 3:10–12, 23–24; see Hebrews 6:1–8). This act is a serious sin against Christ (Galatians 5:2–4; compare Hebrews 6:4–6). See also study note on 2:19.
If I rebuild: This refers to insisting on following Jewish laws about food and circumcision (Galatians 2:11–13).
When Jesus died on the cross, he ended the old system of religious laws that separated Jewish and non-Jewish people from each other and from God (Ephesians 2:14–16). His death created a new way for all people to come to God through faith.