ἐξεκλείσθη … διὰ νόμου πίστεως

Says 3:27: Ποῦ οὖν ἡ καύχησις; ἐξεκλείσθη. διὰ ποίου νόμου; τῶν ἔργων; οὐχί, ἀλλὰ διὰ νόμου πίστεως (BSB "Where, then, is boasting? It is excluded. On what principle? On that of works? No, but on that of faith"). ἐξεκλείσθη is aorist passive: boasting "was shut out." Abbott-Smith glosses καύχησις "a boasting, glorying" (Rom 3:27).

Says The boasting recalls 2:17, 23, where the Jew boasts in God and in the law.

Uncertain What νόμος means in νόμου πίστεως (nomou pisteōs). This note leans to "principle" or "rule" (BSB), since διὰ ποίου νόμου asks by what kind of rule boasting is excluded. The main alternative is the Mosaic law itself, viewed in relation to faith (compare 3:31; 9:31–32).

Implies Boasting is excluded because justification is a free gift received by faith (3:24, 28; 4:2–5).