Do you know what the most beautiful diamond in the world looks like in a dark room? Actually, you can't even see it. A diamond has no brilliance or glory of its own. It only shows "its brilliance" when light is shown into it. So, in fact, the diamond has no brilliance; it only has the ability to reflect brilliance. When you shine a bright light into a diamond it displays a glorious spectrum. When laser pointers became a thing back in the 90's I would take the kids into a dark room and shine the laser into Janine's ring. It became a spectacular light show, until the laser was turned off, then there was darkness.
To continue to the thought, have you ever seen a diamond in the rough? If you don't know what you are looking for, you would probably never see it. God knows that for which He is looking. He carves His people out of this world (it's called discipling in the New Testament), and cuts the diamond in just the right way so that each diamond is able, in the way God has designed it to do so, can reflect the glory of Christ. Every diamond is different, as is every Christian. But the light that goes into the diamond is the same - pure and radiant.
When you look at all the wonderful blessings that Peter describes as belonging to the Christian in 1 Peter 2, he says that we have all these blessings so that you may proclaim the praises of Him who called you out of darkness into His marvelous light. Why are you a chosen generation, a royal priesthood, a holy nation, His own special people? So that we can show the glory of the One who has chosen us to be His genetic offspring ... so that we can show the glory of the One who gives us a royal bloodline ... so that we can show the glory of the One who makes us a part of His holy ethnicity ... so that we can show the glory of the One who has by the blood of Christ acquired us as His own people.
The pure light of Christ is poured into us so that we can bring His light into a world of people who live in darkness. True, there will be people who will not comprehend the light ... they will shy away from it because it is different from the darkness to which they have become accustomed. But on the other hand, there are those out there who are searching for light and simply don't know where to turn. You, as the one who has obtained mercy, are to be the lighthouse of God to those who are still struggling to find their way.
Peter's instructions and encouragement come to people who are living their lives in this world. He didn't call us to be monks who separate themselves from culture and live in caves or convents. He called us to live as taxi drivers, real estate agents, janitors, presidents, policemen, housewives, husbands ... et al, with the light of Christ shining through us so that the world in which we live can see His glory.