There is an interesting passage is Isaiah, right toward the end, where the Lord explains that he will choose “priests” and “Levites” from among the Gentiles.
This is remarkable because in order to be a Levite, one must be born of the tribe of Levi. Moreoever, to be a priest (kohan), one must be not only a Levite but a descendent of Aaron. Here the Lord says that he shall choose men who are not Levites, Aaronites, nor even Jews!
I will set a sign among them. And from them I will send survivors to the nations, to Tarshish, Put, and Lud, who draw the bow, to Tubal and Javan, to the coastlands afar off, that have not heard my fame or seen my glory; and they shall declare my glory among the nations.
And they shall bring all your brethren from all the nations as an offering to the LORD, upon horses, and in chariots, and in litters, and upon mules, and upon dromedaries, to my holy mountain Jerusalem, says the LORD, just as the Israelites bring their cereal offering in a clean vessel to the house of the LORD.
And some of them also I will take for priests and for Levites, says the LORD. (Is 66:19-21)
The Catholic Church teaches that this promise is fulfilled by the fact that God has called Gentiles to be his priests and Levites. The priesthood and Levitical ministry is fulfilled in the presbyteral vocation and the diaconal vocation, respectively. Beginning with the Apostolic Fathers, deacons were considered to function like “Levites” because they were clerical but not priestly.
The fact that these Gentile priests and Levites bring a “cereal” offering of wheat is fulfilled by the fact that New Covenant priests offer only the Todah (“Thanksgiving”) sacrifice of Bread and Wine, like Melchizedek.
[This is from a forthcoming book on the subject.]