The term "royal priesthood" (or "kingdom of priests" in some translations) appears explicitly in two places in the Bible:
* Exodus 19:6: "and you shall be to me a kingdom of priests and a holy nation.’ These are the words that you shall speak to the people of Israel." (ESV)
* 1 Peter 2:9: "But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light." (ESV)
In Exodus 19:6, God is speaking to the Israelites at Mount Sinai, promising them a special relationship with Him if they obey His covenant. This signifies a nation set apart for God's purposes, with a unique role in mediating between Him and the rest of the world.
In 1 Peter 2:9, the Apostle Peter applies the concept of a "royal priesthood" to Christians, the believers in Christ. He emphasizes that they are now the chosen people, set apart to declare God's praises and serve as mediators in their own way, bringing others to Christ.