You are a royal priesthood and a holy nation.” I’m combining these two because they are closely related. Peter seems to have taken these two titles and the next one (“a people of his own possession”) from Ex. 19:5-6: “Now therefore, if you will indeed obey my voice and keep my covenant, you shall be my treasured possession among all peoples, for all the earth is mine; and you shall be to me a kingdom of priests and a holy nation…" This is what God said as a promise before He entered into covenant with Israel at Mount Sinai. As you can see, Peter collapsed “a kingdom of priests” into “a royal priesthood.” And the two titles seem to refer to the same thing: a kingdom of priests is a holy nation. So, for the sake of time, I’ll focus on our title as “a royal priesthood.”
This is also a remarkable designation for Christians. Not only are we called priests; we are called royal priests. In this designation, kingship and priesthood are joined together. Few people in the Old Testament had those two offices at the same time. Adam as our covenant representative had all three offices of prophet, priest, and king. And there was that mysterious figure, Melchizedek, who was both a king and a priest. We can say that the Patriarchs--Abraham as well as Isaac and Jacob—had those offices in a very limited sense. After them, Moses had all three offices, too, if only temporarily and functionally without the official titles of “king” or “priest.” David did much to help organize the temple worship but he did so as a king in a theocracy, in which church and state were closely connected together. But he was not a priest in any official sense. And what happened when King Uzziah in his pride tried to offer incense in the temple, which was an official priestly function? God struck him with leprosy and he had to live with it for the rest of his life!