It isn't new. It is how Abraham lived. As was said, righteousness has always been through faith. For it is written, "The just (righteous) shall live by faith." Knowing that a man is not justified (made, righteous) OUT OF the works of the law, if not THROUGH the faith of Jesus ChristI think the larger context is important here because I do not see Paul introducing a new principle of justification in Galatians 2:16 that is different from the one he explains through Abraham in Galatians 3 and Romans 4. The controversy in Galatians concerns people seeking to add works of the law, particularly circumcision, to the means by which a man is justified. Paul says, “we have believed in Jesus Christ, that we might be justified” (Galatians 2:16). Only a few verses later, he explains this by going directly to Abraham: “Even as Abraham believed God, and it was accounted to him for righteousness” (Galatians 3:6).
Verse 17 adds a condition. It states that this justified, being made righteous is undetermined. How do we know? Because if one is seeking to be justified, made righteous they are not justified, made righteous yet. They are a work in progress. If they are found sinners, they are transgressors. If one is crucified with Christ and Christ lives in them and the life they live is through the faith OF the Son of God who gave Himself for them. They don't live by the Law but by faith. They are not continuing in sin that grace might abound. They are dead to sin through the faith OF Christ being lived through them. And if one is dead to sin through the resurrected body of Christ they are dead to the Law. The Law no longer dictates their actions, the transformed heart does which has the Law, God's Word written in it through Christ. We are new creation through His Spirit. That is the word of faith in which we are to preach. (Rom 10:8)
This is the faith Abraham had, He hearkened unto the voice of the Lord. He heard and obeyed the faith he had that God gave Him and received the promises. Even as we receive the Spirit through the hearing of this faith God has given us through Christ living in us.
Gal 3:5 He therefore furnishing you the Spirit, and working the powers in you, is it from the works of the law, or from the hearing of faith?
Gal 3:6 Even as Abraham believed TO THE God, and it was accounted to him for righteousness.