Isaiah 66:21 - Ellicott's Commentary On The Whole Bible

I will also take of them for priests... — We are left to determine whether the promise is that even Gentile converts should be enrolled among the priests and Levites of the new Jerusalem, or that Israelites of the non-priestly tribes should be so enrolled. Was the prophet breaking down in thought the middle wall of partition, or clinging to its maintenance? Isaiah 61:6 seems in favour of the latter view, and we are probably right in looking on this thought, that of all Israel being eligible for the priesthood, as that which was in the prophet’s mind. Like other such thoughts, however, it was capable of expansion, so as to include the whole Israel of God, who were by faith the children of Abraham. (Comp. 1 Peter 2:5; 1 Peter 2:9, with Exodus 19:6.)