Image source: whisper.sh
1 Peter 2:9, KJV: "But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light:"
As Israel was, we—believers, the church—are also a chosen race. Unlike Israel, though, we are not a race in the sense of our family, ethnicity, skin color, or country of origin. We are a spiritual race, in the sense that, in Christ, we share a single spiritual Father. In that same meaning, we are a "holy nation," a specific group of people called out and set apart from all other nations.
Christians, together, are a royal priesthood. In Israel, under the Law, one tribe was given the task of serving as priests, performing the duties of mediators between God and the rest of the nation. But in and through Christ, the King, all believers are priests with direct access to our Father. We need no other mediator.
Finally, God has formed this nation, this race, this priesthood to take possession of us. We are His people in a very real way. It's not just that we pledge our allegiance to Him; it's that He has taken ownership of us. We belong to Him.
Why? For one, He has made us to declare His praises. That's more than just a natural response to our Father's mercy or something we should do to be polite. Declaring God's greatness is one reason for our very existence. When we praise Him, we fulfill our purpose. He has called us out of the darkness of a meaningless life, and an eternity apart from Him, and into His wonderful, marvelous light. Because we belong to Him, we belong in His light.