It is pure invention that pope, bishops, priests and monks are called the spiritual estate, while princes, lords, artisans, and farmers are called the temporal estate. This is indeed a piece of deceit and hypocrisy. Yet no one need be intimidated by it, and for this reason: all Christians are truly of the spiritual estate, and there is no difference among them except that of office.[LW 44:127].
It follows from this argument that there is no true, basic difference between laymen and priests, princes and bishops, between religious and secular, except for the sake of office and work, but not for the sake of status. They are all of the spiritual estate, and are truly priests, bishops, and popes. We are all one body of Christ the Head, and all members one of another. Christ does not have two different bodies, one temporal, and the other spiritual. There is but one Head and one body.[LW 44: 129-130].
To be sure, Luther goes a little far in his response to the errors of Rome. The offices of bishop/elder and deacon are real offices, not merely labels. And the elders and deacons are to serve the Body of Christ (whose only head is Christ) in unique ways that are not the same as the duties of all the other members of the Body of Christ. Nevertheless, the basic point that Luther is making is correct: there is one head over the Body of Christ, and it is Christ not a pope.
Likewise, all men (whether peasants or kings) have a responsibility to promote the true religion of Jesus Christ according to the abilities, gifts, and opportunities that God has given them, and in their functions within the kingdom of this world.