Galatians 2:15-21

v15-18: No-one is justified, that is, declared righteous in God's sight, by obeying the law.  This is because no-one, apart from Christ, has perfectly obeyed the law.  Justification is only through faith in Christ who gives to us His righteousness.

v19-21: By faith a Christian has been united to Christ, "by means of which he shares in the death of Christ and, by virtue of that death, dies to sin, but also shares in the life of Christ, and by virtue of that life, lives to God." Those who have faith live by that faith, "and the great thing that faith focuses on is Christ's loving us and giving himself for us... and this is what we are chiefly concerned to have faith in, in order to live to him."