The Apostle Peter made this statement:
4) Coming to Him as the living stone, rejected indeed by men, but chosen by God and precious, 5) you also, as living stones, are being built up a spiritual house, a holy priesthood to offer up spiritual sacrifices acceptable to God through Jesus Christ. 6) Therefore it is also contained in the scripture, "Behold, I lay in Zion, a chief cornerstone, elect, precious, And he who believes on Him will by no means be put to shame. 9) But you are a chosen generation, a royal priesthood, a holy nation, His own special people, that you may proclaim the praises of Him who called you out of darkness into His marvelous light; 10) who once were not a people but now the people of God, who had not obtained mercy but now have obtained mercy. (2 Peter 2:4-6,9-10)
Also read Psalm 18:22
Throughout human history, the religious society has generally separated priest and king into separate offices and functions but God did not design them that way in the beginning. When God created us. He intended for us to be His representative rulers, His ambassadors, over the rest of the created order. We were to be priests, kings of the Earth; as priests representing God's nature and character, and as kings, His Kingdom government.
Through Christ we are a "holy priesthood" a "chosen people" a "royal priesthood" and a "holy nation". As such, we have been restored to our priestly function of representing and reflecting God's nature and character before the world. We are also the "people of God" not a nation of subjects, but of sons and daughters. If God is a King, then we, His people, are also of the royal line. Therefore, we have also been restored to our kingly function of representing the government of God on the Earth.
Keep in mind, God is not interested in having subjects in His Kingdom. He wants only children, royal heirs to His treasures of His domain. Our mission as ambassadors of the Kingdom of God is to bring those who are enslaved in the Kingdom of darkness to Christ, the door, so that He can set them free to enter into their full citizenship in God's kingdom of light.