Adam Clarke on Galatians 2:16

Knowing that a man is not justified - See the notes on Rom 1:17; Rom 3:24 (note), Rom 3:27 (note); Rom 8:3 (note). And see on Act 13:38 (note) and Act 13:39 (note), in which places the subject of this verse is largely discussed. Neither the works of the Jewish law, nor of any other law, could justify any man; and if justification or pardon could not have been attained in some other way, the world must have perished. Justification by faith, in the boundless mercy of God, is as reasonable as it is Scriptural and necessary.