Luther has a fascinating discussion on the priesthood of all believers, in his Right and Power of a Christian Church, where he discusses the layperson’s duty to preach (even if he has not been called to the vocation of a preacher). I think the implications for mission are obvious, but rather than saying too much about it, I’ll let Luther speak for himself. [Note: The context in which the passage arises is that of all Christians’ duty to judge their pastors, and their right to call their own pastors.]

No one can deny that every Christian has God’s Word and is taught by God and annointed by him to the priesthood. Thus Christ says in John 6 [:45], “They shall all be taught by God.” And in Psalm 45 [:7], “God has anointed you with the oil of gladness above your fellows.” By “fellows” are meant Christians, Christ’s brethren, consecrated to be priests with him. As Peter also says in 1 Peter 2 [:9], “You are a royal priesthood, that you may declare the wonderful deeds of him who called you into his marvelous light.”

Now, if Christians have the Word of God and are anointed by him, they are in duty bound to confess, preach, and spread this Word. It is as Paul says in II Corinthians 4 [:13], “We have the same spirit of faith, and therefore we speak.” The prophet says in Psalm 116 [:10], “I believed, therefore have I spoken,” and in Psalm 51 [:13] he says in the name of all Christians, “I will teach transgressors thy ways, and sinners will return to thee.” These passages prove once more that a Christian not only has the right and power to teach God’s Word but is in duty bound to teach it on pain of losing his salvation and forfeiting God’s favor.

But you will say, “How is he to do this? For unless he has been called to do this he dare not preach, as you yourself have repeatedly taught!”

I reply: Here you must consider the Christian from a double point of view. On the one hand, if he is in a place where there are no Christians, he needs no other call than the fact that he is a Christian, inwardly called and anointed by God; he is bound by the duty of brotherly love to preach to the erring heathen or non-Christians and to teach them the gospel, even if no one has called him to this work. That is what St. Stephen did (Acts 6 and 7); the office of preaching was not committed to him by the apostles, yet he preached and performed great wonders among the people. Philip, Stephen’s fellow-deacon, did the same (Acts 8 [:5]) without having received the office of preaching. The same is true of Apollos (Acts 18 [:25-26]). In such circumstances the Christian looks in brotherly love on the needs of poor, perishing souls and waits for no commission or letter from pope or bishop. For necessity breaks every law and knows no law; moreover, love is bound to help where there is no one else to help.

On the other hand, if a man is in a place where there are other Christians who have the same power and right that he has, he should not thrust himself forward but should rather let himself be sought out and called to preach and teach in the stead and by the commission of the rest. Even among other Christians a Christian has the right and obligation to get up and teach without being called by men if he should find the teacher in that place to be in error, provided that this is done in a becoming and decent manner. Such a case is plainly described by St. Paul in I Corinthians 14 [:30], where he says, “If a revelation is made to another sitting by, let the first be silent.” Notice what St. Paul does here. He commands the man who is teaching to be silent and to step aside (among Christians!) and commands the hearer to speak up, even without a call, because necessity knows no law.

…In the same passage St. Paul gives every Christian the right to teach among Christians whenever it becomes necessary: “You can all prophesy one by one, so that all may learn and all be encouraged” (I Corinthians 14 [:31]), and, “Desire earnestly to prophesy, and do not forbid speaking in tongues; but all things should be done decently and in order” [I Cor. 14:39-40]. Take this passage as a very sure ground which gives more than sufficient authority to the Christian community to preach, to permit men to preach, and to call preachers. Especially in case of necessity this passage itself summons each and every one without any call of men.

A powerful passage, no? It is certainly helpful in clarifying the meaning of Article 14 of the Augsburg Confession : “Our churches teach that no one should publicly teach in the Church, or administer the Sacraments, without a rightly ordered call.” There are occasionally instances when any Christian must by necessity preach.

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(The passage from The Right and Power of a Christian Church or Community to Judge All Teaching and to Call, Appoint, and Dismiss Teachers, Established and Proved from the Scriptures is selected from Steinhaeuser’s translation, as revised by Tappert in Selected Writings of Martin Luther: Volume 2. Fortress Press, 2007.)