In Exodus 19:6, Yahweh says that He is making Israel a “kingdom of priests.” In the context of Scripture to this point in the story, we only have three examples of “priests”: Melchizedek priest of Salem, Potiphera priest of On in Egypt, and Jethro priest of Midian.
This creates a striking picture of “priests.” So far priests are all gentiles, outsiders, God-fearers from a distance. And all three are instrumental in providing rest for the people of God. Melchizedek provides a feast of bread and wine and blesses Abraham after his battle with the five kings. Potiphera gives his daughter in marriage to Joseph, and the priests of Egypt are at least in the background of Joseph’s care for his family and the rest of the nation of Egypt (cf. Gen. 46-47). Finally, Jethro (like Potiphera) gives his daughter in marriage to God’s appointed deliverer, Moses (like Joseph), and it is Jethro who shows up after the battles with Pharaoh and Amalek to eat bread with Moses and the elders of Israel (like Melchizedek). And Jethro gives Moses counsel for organizing the people so that they might “go to their place in rest” (Ex. 18:23).
If we consider Joseph a sort of extension of the ministry of the priests of Egypt, all three are significant for the bread they share with the people of God, for the rest they give during hard times.
When Yahweh says that He is making Israel a kingdom of priests, He means that He is making Israel a nation of Jethros, a kingdom of Melchizedeks, a family of Josephs who have bread and sabbath for the world.
[Insert typological significance for Christ as priest according to order of Melchizedek.]