Jamieson, Fausset and Brown on James 2:8

8. The Greek may be translated, "If, however, ye fulfil," &c., that is, as Alford, after Estius, explains, "Still I do not say, hate the rich (for their oppressions) and drive them from your assemblies; if you choose to observe the royal law ... well and good; but respect of persons is a breach of that law." I think the translation is, "If in very deed (or 'indeed on the one hand') ye fulfil the royal law ... ye do well, but if (on the other hand) ye respect persons, ye practice sin." The Jewish Christians boasted of, and rested in, the "law" (Ac 15:1; 21:18-24; Ro 2:17; Ga 2:12). To this the "indeed" alludes. "(Ye rest in the law): If indeed (then) ye fulfil it, ye do well; but if," &c.

royal--the law that is king of all laws, being the sum and essence of the ten commandments. The great King, God, is love; His law is the royal law of love, and that law, like Himself, reigns supreme. He "is no respecter of persons"; therefore to respect persons is at variance with Him and His royal law, which is at once a law of love and of liberty (Jas 2:12). The law is the "whole"; "the (particular) Scripture" (Le 19:18) quoted is a part. To break a part is to break the whole (Jas 2:10).

ye do well--being "blessed in your deed" ("doing," Margin) as a doer, not a forgetful hearer of the law (Jas 1:25).