Let the richness of that statement settle in:
You are a member of a chosen people. This goes way beyond a personal relationship with Jesus Christ. The group of chosen people is a royal priesthood and a holy nation. It's called out and set apart by God - not to get private benefits for themselves, but to give public blessings to others. This goes way beyond any particular flag, national anthem, or pledge of allegiance - this is global. The group belongs to God. It's God's possession - bought at a price. This goes way beyond pot-luck suppers and pizza parties. It's about the full, deep realities of the cosmos. And the group exists to declare the praises of God. This goes way beyond Sunday morning. It's about your whole life, so that all your waking and resting, eating and drinking, working and playing can be offered to God as acts of praise and worship.
But here's the catch: this vocation is not original to Christians in the New Testament, it's an echo of Israel's vocation in the Old Testament.
"You yourselves have seen what I did to Egypt, and how I carried you on eagles' wings and brought you to myself. Now if you obey me fully and keep my covenant, then out of all nations you will be my treasured possession. Although the whole earth is mine, you will be for me a kingdom of priests and a holy nation" (Exodus 19:4-6 NIV).
How did this text make it from the Old Testament Israelites to the New Testament Christians? That's part of the long story the Bible is telling.
God created. Man rebelled. Sin spread.
God promised. Pharaoh enslaved. God delivered.
God offered. Israel tried.
Kings ruled. Prophets threatened. Babylon conquered.
Exile started. People returned.
And they waited...
Rather than embracing their God-given identity through obedience to God and blessing to the nations, Israel chose the path of folly, arrogance, and disobedience. So instead of nations being blessed by Israel, the nations mocked Israel.
And so they waited...
for God to become their king again, for God to dwell in their midst again, for God to implement the new covenant spoken about by the prophets Jeremiah and Ezekiel.
Then out by the Jordan River, a fiery prophet named John, who is known as the baptizer, starts gathering a people to prepare the way for the one who is to come - the Messiah, the King of the Jews, and the Lord of the whole world.
Jesus emerges as the one they've been waiting for. He is the one to whom all the signposts of the Old Testament are pointing - David's son, the suffering servant, the Son of man, the Temple, the sacrifice, the Word and Wisdom of God. Jesus is presented as the one who fulfills and upstages them all - most notably on the cross and through the resurrection when new creation sprang to life in the midst of the old.
Then we find ourselves back again at 1 Peter 2:9. Now, with the volume amplified, hopefully the echo can be heard more clearly. The identity of "God's people advancing God's purpose" was assigned to Israel in the Old Testament, but their particular role was fulfilled by Jesus and has now expanded to include the worldwide web of Jesus' followers who are gathered and scattered in churches comprised of people from every nation, tribes, and tongue.
"Jesus came to them and said, 'All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age'" (Matthew 28:18-20 NLT).
Keeping It Together: How To Read the Bible
Creation, Vocation, Permission, & Prohibition