James 2:8

The apostle, having condemned the sin of those who had an undue respect of persons, and having urged what was sufficient to convict them of the greatness of this evil, now proceeds to show how the matter may be mended; it is the work of a gospel ministry, not only to reprove and warn, but to teach and direct. Col 1:28, Warning every man, and teaching every man. And here, I. We have the law that is to guide us in all our regards to men set down in general. If you fulfil the royal law, according to the scripture, Thou shalt love thy neighbour as thyself, you do well, Jam 2:8. Lest any should think James had been pleading for the poor so as to throw contempt on the rich, he now lets them know that he did not design to encourage improper conduct towards any; they must not hate nor be rude to the rich, any more than despise the poor; but as the scripture teaches us to love all our neighbours, be they rich or poor, as ourselves, so, in our having a steady regard to this rule, we shall do well. Observe hence, 1. The rule for Christians to walk by is settled in the scriptures: If according to the scriptures, etc. It is not great men, nor worldly wealth, nor corrupt practices among professors themselves, that must guide us, but the scriptures of truth. 2.

— Matthew Henry's Concise Commentary (public domain)

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" (Act 15:1; Act 21:18-24; Rom 2:17; Gal 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 (Jam 2:12). The law is the "whole"; "the (particular) Scripture" (Lev 19:18) quoted is a part. To break a part is to break the whole (Jam 2:10).

— Jamieson-Fausset-Brown Commentary (public domain)