Weigle, L. A. (1920). The teacher's knowledge of the lesson. In L. A. Weigle, Talks to Sunday-school teachers (pp. 107-113). New York, NY, : George H. Doran Company.
http://dx.doi.org/10.1037/14756-014
Since becoming a teacher, I have found out that the easier task is that of the student. The teacher must know so much more and must study so much harder than I had thought. Every class makes demands upon its teacher far greater than any that he can make upon It. The teacher must not simply know the lesson; he must know it in such a way that he can cause others to know it. That is much more difficult. It means that he must understand his pupils as well as the lesson Itself, and that he must, be able to present it clearly and in such fashion as to arouse their interest, command their attention and set them to work upon it. He will fall if he be lacking in any one of three respects: in his mastery of the subject, in his understanding of his pupils, or in his ability to bring the two together. We are now to think only of the first of these qualifications of the teacher; in subsequent chapters we shall think of the other two. The teacher's mastery of the lesson material cannot be too complete and thorough. "Oh, I cannot teach a Sunday school class, I do not know enough" is perhaps the most frequent excuse given to pastors and superintendents who are seeking new teachers. And all too often the statement is true. The person asked does not know enough to be a good teacher, without a definite course of preparation, not simply in psychology and pedagogy, but in the subject-matter itself which is to be taught. But that is no reason why he should not accept the responsibility and undertake courses of study which will in time cause him to know enough. (PsycINFO Database Record (c) 2016 APA, all rights reserved)