Peter, who wrote two (maybe 3) books in the New Testament, encourages the believers scattered through Asia. He describes them as "the spiritual house of God," and "a holy priesthood" in 1st Timothy 2:5.
We take it for granted that it must be true; because, I mean, it's in the Bible.
But, what we so often forget is that it wasn't always in the Bible. So, one of the questions that we are encouraged to ask as followers of the Biblical Messiah, is why is it there, and what did Peter intend when putting it there.
A priest isn't just someone who knows a lot about God and does special God-stuff. His role is to bridge the gap between the carnal, the clueless and the Creator. His role is that of in intermediary or go-between, a tour guide, and a match-maker. The priest offers the possibility that God exists to a world that otherwise would not see the spiritual realities for what they are. The priest's role is to demonstrate faith to a people who can't otherwise see past the ends of their noses.
In Exodus 16:9, the nation of Israel was commissioned for this role. In Isaiah 56:8, we are also told that His plan for dispersing Israel among the nations was so that others might, too, be exposed to the Holy God of Israel. The role of priests is never introspective or self-purposed. It is always as a light toward others. Peter sees the gentile believers (not-Jewish) commissioned to fill this very same role, as "grafted into the root of Israel" (Romans 11). So, "the Priesthood of the Believer" is not an idea that started with John Calvin in 1536 ACE/AD. It started much earlier, possibly going back as far as Peter, written at or around 60 Ace/AD; or even Moses penned the book of Deuteronomy in 1422 BCE/BC. The idea, itself, is sparked when God first makes His covenant with Abraham around 1762 BCE/BC, when He says to Abraham, "through you shall all the nations of the earth be blessed." Then later, to Jacob, when he awoke from his dream, God commands him to look to the West, to the East, to the North and to the South; a phrase which is repeated to Moses in our portion of Veh etchanan. So, while the instruction is given specifically to the nation of Israel, it expands much farther: to everyone who calls upon the God of Israel (the God of Abraham, Isaac and Jacob).
Veh etchanan is an appeal to those who have come to know the God of Israel in a real and powerful way. It calls to us to live out the experience of that relationship as a beacon of truth and righteousness. It literally boils down to this: Be the Bible that the world around you needs to know. Of course, it doesn't stop with that. It outlines for us some very specific ideas of what the world needs to see from us. The question before us today is whether we will choose to live as the priesthood that will bring others to us or to establish ourselves as a priesthood for our own benefit. But this is a question that every priesthood will wrestle with until the Messiah establishes His Kingdom.