The Priesthood of All Believers

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 marvelous light (1 Peter 2:9 ESV)

Luther formulated the doctrine of the “priesthood of all believers" in the early years of the Reformation. In To the Christian Nobility of the German Nation (1520) he wrote that all Christians are equal before God regardless of whether they are clergy or laity:

The Pope, bishops, priests, monks and nuns are called the “spiritual class" while princes, lords, artisans and peasants are called the “secular class". This is a deceptive invention but no one should be concerned, for all Christians are truly of the spiritual class and there is no difference among them except in the work they do... for baptism, the gospel and faith alone make a spiritual and Christian people

Luther also reinforced the view that Christians exist in community and that the power to call individuals to ministry resides in this community:

Because we are all priests of equal standing, no one must push himself forward and take it upon himself, without our consent and election, to do that for which we all have equal authority. For no one dare take upon himself what is common to all without the authority and consent of the community

For a more detailed overview see “The Early Luther on Priesthood of All Believers, Office of the Ministry and Ordination" by Cameron MacKenzie of Concordia Theological Seminary.