To the Christian Nobility of the German Nation, published in August 1520, is a political tract as much as a theological one, addressed to the lay rulers of the empire and arguing that they were entitled to reform a church that would not reform itself. Luther framed the obstacle as three walls. The first was the claim that the spiritual estate, meaning pope, bishops, priests and monks, stands above the temporal estate of everyone else and is not subject to it. The second was that only the pope may interpret scripture. The third was that only the pope may summon a council. He attacked the first as the one holding up the other two, and his instrument was baptism.
All Christians are of the spiritual estate by baptism and the gospel, differing only in the work they are called to do, so a cobbler, a smith or a farmer is as much a consecrated priest as any bishop; what a priest has that a layman does not is an office in the congregation, held on the congregation's behalf. Luther drew the language from 1 Peter 2, where the whole people is called a royal priesthood, and from Revelation 1 and 5, where the redeemed are made priests to God. The consequence he pressed hardest was practical rather than metaphysical: if the difference is one of office and not of spiritual kind, then a council can be called by whoever is able to call one, and scripture belongs to whoever can read it.