John Calvin Commentary Exodus 19:6

"and ye shall be unto me a kingdom of priests, and a holy nation. These are the words which thou shalt speak unto the children of Israel." — Exodus 19:6 (ASV)

And you shall be to me. He points out more clearly, and more at length, how the Israelites will be precious to God; namely, because they will be for a kingdom of priests, and a holy nation. By these words, he implies that they will be endowed with sacerdotal as well as royal honors, as if to say that they would not only be free but also like kings if they persevered in faith and obedience, since no kingdom is more desirable or happier than to be the subjects of God.

Moreover, he calls this a holy kingdom because all the kingdoms of the world were then in heathenism. For the genitive, according to the usual idiom of the language, is used for an adjective, as if he had said that they would enjoy not merely an earthly and transitory dominion, but also a sacred and heavenly one. Others understand it passively: that God would be their king, while mortals, and for the most part cruel tyrants, would rule over other nations.

Though I do not altogether reject this sense, I rather prefer the other, to which Saint Peter also leads us. For when the Jews, who by their refusal of Christ had departed from the covenant, still improperly gloried in this title, he claims this honor for the members of Christ only, saying, You are a chosen generation, a royal priesthood, etc. (1 Peter 2:9). But the passive sense would not accord with these words—namely, that believers are subject to the priesthood of God—for the Apostle gracefully applies the words to take away the unacceptability of novelty, as if he had said, God formerly promised to our fathers that they should be to Him for a royal priesthood.

All who separate themselves from Christ the Head falsely lay claim to this privilege, since He alone makes us a royal priesthood. Meanwhile, he teaches by this apparent adaptation of the words that what had been spoken by Moses is actually fulfilled. Indeed, Christ appeared invested with the kingdom and the priesthood, that He might confer both of these privileges upon His members. From this it follows that whoever divorces themselves from Him are unworthy of either honor and are justly deprived of them.

The nation is here called holy, not with reference to their piety or personal holiness, but as set apart from others by God by special privilege. Yet on this kind of sanctification, the other depends: namely, that those who are exalted by God’s favor should cultivate holiness and thus, on their part, sanctify God.