Isaiah 66:21 - Albert Barnes' Notes on the Bible

And I will also take of them for priests - I will give to them an honorable place in my public service; that is, I will make them ministers of religion as if they were priests and Levites. This cannot be taken literally - because the priests and Levites among the Jews were determined by law, and by regular genealogical descent, and there was no provision for substituting any in their place. But it must mean that under the condition of things described here, those who should be brought from the distant pagan world would perform the same offices in the service of God which had been performed formerly by the priests and Levites that is, they would be ministers of religion. The services of God would no longer be performed by the descendants of Aaron, or be limited to them, but would be performed by others who should be called to this office from the pagan world.