Dr. Nagel assumes his readers would be quite familiar with the Augsburg Confession, article 5. Here it is. Article V: Of the Ministry.
That we may obtain this faith, the Ministry of Teaching the Gospel and administering the Sacraments was instituted. For through the Word and Sacraments, as through instruments, the Holy Ghost is given, who works faith; where and when it pleases God, in them that hear the Gospel, to wit, that God, not for our own merits, but for Christ's sake, justifies those who believe that they are received into grace for Christ's sake. They condemn the Anabaptists and others who think that the Holy Ghost comes to men without the external Word, through their own preparations and works.
Now that we've read that, where is Dr. Nagel going with his article? At the time of the Reformation there were many radical reformers setting themselves up in ministry. They were not called in a recognizable manner, asserting that all believers are ministers of the Gospel. And many of them were not receptive to training or corrective discipline. This sounds remarkably similar to many situations we hear about today.
We observe that not only does the Augsburg Confession state what is right, it also says what is wrong. To simply say what is right is inadequate. It fails to issue correction. The result is that some people can hold contradictory points of view and not see that they are contradictory. For instance, a believer may think that God works through his external word (externum verbum) and also calls some without that. A believer might think that God saves those who live a godly life as well as some who didn't live a particularly godly life but believed. Without stating a right doctrine and a wrong doctrine, there will be some who hold to their wrong doctrine while purportedly embracing the right doctrine.
Nagel points to the supernatural work which is engaged in by the minister of the Gospel. He is called and ordained appropriately, receives training and discipline, and works knowingly. And when he proclaims the word of Christ it is not he who speaks, but Christ. God accomplishes His will through his Word, both the spoken word, and the living Word, Jesus Christ, present in the Sacraments. And he accomplishes this will not through out goodness, not through our willingness to hear and obey, but by his mercy and grace, working in us though we are dead and bent toward sin. There we who find we cannot perform enough of our own preparations or works find hope and confidence.