“But you are a chosen race, a royal priesthood, a holy nation, a people for His own possession, that you may proclaim the excellencies of Him who called you out of darkness into His marvellous light” (1 Peter 2:9).
The Church is God’s chosen race, though multitudinous with every race on earth. She consists entirely of priests, and she is a holy nation—God’s own possession. Just as God chose the Levite priests for Himself (Numbers 3:11-12), so He has chosen every Christian as priests for Himself. In what way is every Christian a priest, however? Not everyone is a pastor. Luther explains it well:
But after we have become Christians through this Priest and His priestly office, incorporated in Him by Baptism through faith, then each one, according to his calling and position, obtains the right and the power of teaching and confessing before others this Word which we have obtained from Him. Even though not everybody has the public office and calling [of pastor], every Christian has the right and the duty to teach, instruct, admonish, comfort, and rebuke his neighbor with the Word of God at every opportunity and whenever necessary.
LW 13:333; emphasis mine
The world hates us when we do this, of course. When we warn them of their sin, they tell us to mind our own business, while with hypocritical vanity they try to tell the Church whom we should marry and what we should teach. Yet each Christian is a priest according to his or her vocation. For example, the father and mother scold their children when they misbehave and forgive them when they repent. When a friend is living in sin, we give them that necessary Law while never forsaking the Gospel. When the government publicly endorses the killing of unborn babies, as citizens Christians admonish them to stop this evil. Yes, Christians help keep the government in check, lest they oppress the people and subsequently suffer the wrath of God. As priest, the Christian always speaks against evil; he does not muzzle his mouth with moral cowardice.
By its very nature, the Word of God is meant to be spoken, especially by God’s people. If God’s Word is not on your lips, how can you call yourself a Christian? This is what God has given every Christian to do in their given vocations—to speak the Word of God in Law and Gospel. Not every Christian is a pastor—that is, a called and ordained servant of Christ—but every Christian is a priest in the sense that they speak the Word of God. They speak the Law to hardhearted sinners and the Gospel to the humble seeking mercy. This is our most sacred calling. We mustn’t forsake it and the government cannot prevent us from speaking it, even if it should kill us.
The Lutheran Column 