"It is true: All Christians are priests, but they are not all pastors, for they mus not only be Christians and priests but also be in charge of the office [ministry] and parish. It is the call and command that makes pastors and ministers. Just as a citizen or layman, yet for that reason he is not a doctor [professor], so that he may publicly teach in the schools, nor may he appropriate that office to himself; he must be called to it.
"This I had to say of the sneaky, treacherous preachers, of whom there are now many, to alert the pastors and goverments diligently to warn the people and to command them to take care of such intruders and vagabonds and to guard against them as the devil's own sure messengers. They must show their credentials and proofs that they are sent by God for such work in the parish; otherwise they are not to be admitted and heard, even though they wanted to teach the pure Gospel, indeed even if they were angels and Gabriel himself come down from heaven. For God does not want to have anything done by our own choice or devotion but everything only by [His] command or call, especially in the holy ministry. Thus St. Peter says: 'knowing this first, that no prophecy of Scripture is of any private interpretation, ... but holy men of God spoke as they were moved by the Holy Spirit' (2 Peter 1:20-21). Therefore Christ also did not permit the devils to speak, though they declared Him to be the Son of God and proclaimed the truth (Luke 4:34-35; Mark 1:24-25). He did not want to permit such an example to preach without a call" ("Exposition of Ps. 82," 1530 St. Louis edition, 5:721-11)
In Walther's "Church and Ministry" pp 164-5.