1Peter 2:9 But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should forth the praises of him who hath called you out of darkness into his marvellous light.
The promise to me is that I will show forth the praises of God. As a loved and saved child of God, my life and words will praise God. The good words and good deeds of a Christian are praises to God. God is praised by my life. I by myself was in sinful, ignorant darkness with no life that praises God, or even pleases Him. But God called me out of darkness into His marvelous light. In the light the life of God is reflected in our life. This is the promise: my life will praise God in word and deed for who He is and for what He has done. I am not commanded to praise God; it is a promise that I will, for thanks and praise is the natural response to the goodness, grace, and mercy of God.
Another part of the promise is that the way I lives causes others to praise God. Matthew 5:16: ”Let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven.” A good Christian life draws attention to Jesus and not to us. He gets the glory for any goodness that is seen in us.
It is extremely difficult to live that way. Self-centered sinners want credit for being good. Pride makes it hard to give God the glory and the credit for every good thing we do. So the Lord promises humility and penitence on our part and praise and glory on His part. We are nothing; Jesus is everything. With that in mind He makes us chosen, priestly, holy, and peculiar.