All Christians have great blessings as a consequence of our unity with our Savior. The blessings are described in various parts of of the Bible, but in I Peter 2:9, the blessings are summarized in beautiful and glorious fashion. The verse is so poetic that it is easy to neglect the deep truth in it. Of course, with the Bible as a whole, the context reinforces the doctrine. In this case, the context is a contrast. The Christian's blessings are contrasted with the those described in the previous verse which describes those who were appointed to stumble at Christ and the gospel.
The first contrasting blessing is that believers are a chosen generation. Those who express faith in Christ and received the gospel with joy do so because of the Divine Choice to regenerate them. This word "generation" can refer to those who have a common ancestor. If you are united to Christ, you and all believers share the same genealogy and The Savior is our common "ancestor." "For we are members of his body, of his flesh, and of his bones." (Eph. 5:30)
Secondly, all Christians are part of a royal priesthood. In the previous verses, Peter described those who rejected the true temple and the true priesthood. The doctrine of the priesthood of all believers is a great inheritance we have from the Reformation. But what ought to particularly get your attention is the word "royal". The kings and the priests under the Old Covenant were separated: kings were to be sons of David and priests were to be sons of Aaron. But those who are united to Christ are "partakers of His anointing" (Heidelberg Catechism Q/A 32). The Savior is a "priest forever after the order of Melchizedek" and Melchizedek was the priest-king of Salem. So as the Catechism says, this anointing gives you the great privilege to "present [you]self a living sacrifice of thankfulness to Him, and that with a free conscience [you] may fight against sin and the devil in this life, and hereafter in eternity reign with Him over all creatures."
Thirdly, all Christians are part of a holy nation and are a peculiar people. In this little phrase, Peter has in mind the prophesy from Isaiah 62:10-12. Isaiah prophesies that the redeemed of the Lord would be brought back and that they would constitute a 'holy people". The Holy Spirit, through the Apostle Peter, applies this to all Christians whether Jew or Gentile. What makes a nation a nation? Among other things, a nation has a common language, a common government, a common means of exchange, a common right to self-defense, a common culture, laws in common and a common flag. To a greater or lesser extent, these apply to believers as a type of nation. What makes the nation a holy nation? The nation is a holy nation because it has been set aside particularly for and to God. That is, the holy nation is God's "peculiar" nation. "Peculiar" means "one's purchased possession." Or, as Isaiah 62 puts it, "sought out, a city not forsaken."
And lastly, all of this is in order for us to "show forth" His praises. It was the office of the prophet under the old covenant to declare the words of the God of Israel ‚Äď the only God and Creator of Heaven and Earth. He could to this by foretelling what would come to pass. But this kind of prophesy was only one kind of prophesy. It was also the office of the prophet to declare what God's Word was in relation to the current state of affairs. That is why you often observe in the Prophets that they are prosecuting the covenant people for their sins and covenant breaking. They were presenting God's verdict for their very current sins. They had the authority that comes from one who had direct revelation from God. But for us, the revelation is complete and enscripturated. It is your task to "show forth" or proclaim; to publish and declare the Word of God preserved for you. Your movement from darkness into the marvelous light is all of the motivation you need. Everyday, and in all ways, proclaim His praises.