John Calvin Commentary Isaiah 66:21

"And of them also will I take for priests [and] for Levites, saith Jehovah." — Isaiah 66:21 (ASV)

And I will even take some of them for priests and Levites. The Prophet heightens the description of what he had already declared about the extraordinary grace of God. He had made known that the Church of God would be gathered from all nations, so that, despite every difficulty and obstruction, even distant nations would draw near to them. But now he proceeds further and instructs them that the Gentiles will not only be adopted by God but will also be elevated by him to the highest honor. It was already a great honor that unclean and polluted nations were reckoned to be a holy people; but now here is something far more wonderful: they are elevated to the highest pinnacle of rank.

Therefore, we see that the priesthood under Christ is very different from what it was under the Law. For under the Law, one tribe exclusively was admitted to the priesthood, and the Gentiles, as unclean, were so far from being able to discharge that priesthood that they were even forbidden to enter the temple. But now, all are admitted without distinction. Some interpret this passage generally: that the Gentiles will be priests; that is, they will offer themselves to God, as Scripture frequently designates all believers a royal priesthood (1 Peter 2:9; Revelation 1:6; Revelation 5:10). But he appears to describe, in a special manner, ministers and teachers whom the Lord also chose from among the Gentiles and appointed to carry out this distinguished office—that is, to preach the Gospel—such as Luke, Timothy, and others of the same class, who offered spiritual sacrifices to God through the Gospel.