Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
---II Timothy 2:15
For there must be also heresies among you, that they which are approved may be made manifest among you.
---I Corinthians 11:19
However the churches long ago, in their dead formalism, concretized the matter of teacher certification. You send off a young man to seminary where he learns to teach (or, as we are always told, preach) sermons. The damage is two-fold, for not only are the church's teachers thereby largely limited to seminary graduates, but the sermon–the primary approved vehicle in the churches for teaching–becomes the litmus test of a teacher in the churches of God. And not only so, but a man may or may not teach in the churches only so long as he is "capable of teaching sermons." I.e., a man who teaches outside the sermon format is certified only if he can also teach sermons. By contrast, a man who never teaches sermons but only teaches outside the sermon format is suspect, particularly when he challenges the sermon-system. "I don't need to listen to him, he's not a real preacher."
One of my favorite examples of sound teaching breaks all the rules of Churchianity. It is an address by W. Carl Ketcherside called "The Clergy System." The teacher, Mr. Ketcherside, never attended seminary. He travelled widely as a teacher, just as teachers did in the first century. He regularly challenged the various tenets and traditions embodied in the clergy system. If the reader examines his address, above, he or she will find that it violates the cardinal rule of "expository teaching," so-called, which is that teaching must be based on a single Bible text or a combination of Bible texts. By contrast, Mr. Ketcherside's teaching is based on reading, and understanding, the entire Bible. The teaching is thoroughly Biblical, yet it fails the "sermon test!"
Nevertheless Ketcherside's teaching puts to shame all the sermon-teachers who never, ever, ever, teach the New Testament doctrines and practices of the priesthood of believers. Mr. Ketcherside makes his case without seminary lore or Greek word studies which too often are designed to captivate the audience rather than teach the true meaning of ecclesia, dokimos, laos, klēros, koinōnia, diakonia, leitourgeō, etc. In so doing he upholds the original meaning of the Greek words. Mr. Ketcherside, I take it, was a "strict constructionist" concerning the Word of God. I look forward to meeting the man in heaven one day.