A.T. Robertson Commentary Romans 3:20

"because by the works of the law shall no flesh be justified in his sight; for through the law [cometh] the knowledge of sin." — Romans 3:20 (ASV)

Because (διοτ, again, δια, οτ).

By the works of the law (εξ εργων νομου). "Out of works of law." Mosaic law and any law as the source of being set right with God. Paul quotes Ps 43:2 as he did in Ga 2:16 to prove his point.

The knowledge of sin (επιγνωσις αμαρτιας). The effect of law universally is rebellion to it (1 Corinthians 15:56). Paul has shown this carefully in Ga 3:19-22. Cf. Hebrews 10:3. He has now proven the guilt of both Gentile and Jew.