A.T. Robertson Commentary James 2:8

"Howbeit if ye fulfil the royal law, according to the scripture, Thou shalt love thy neighbor as thyself, ye do well:" — James 2:8 (ASV)

Howbeit (μεντο). Probably not adversative here, but simply confirmatory, "if now," "if indeed," "if really." Common in Xenophon in this sense. See the contrast (δε) in verse 9.

If ye fulfil (ε τελειτε). Condition of first class, assumed as true with ε and present active indicative of τελεω, old verb, to bring to completion, occurring in Ro 2:27 also with νομος (law). Jesus used πληροω in Mt 4:17. James has τηρεω in 2:10.

The royal law (νομον βασιλικον). Old adjective for royal, regal (from βασιλευς king), as of an officer (John 4:46). But why applied to νομος? The Romans had a phrase, lex regia, which came from the king when they had kings. The absence of the article is common with νομος (4:11). It can mean a law fit to guide a king, or such as a king would choose, or even the king of laws. Jesus had said that on the law of love hang all the law and the prophets (Matthew 22:40), and he had given the Golden Rule as the substance of the Law and the prophets (Matthew 7:12). This is probably the royal law which is violated by partiality (James 2:3). It is in accord with the Scripture quoted here (Leviticus 19:18) and ratified by Jesus (Luke 10:28).