But isn't that for two basic reasons?
Number one, if teachers stick to the curriculum, they have at least that to defend themselves with when the pastor comes to pepper them with a battering of Scripture references should a doctrinal or even a merely an interpretative difference arises in class.
Second, even if they love both God and pupils, the Sunday school teacher --- unlike the pastor in most circumstances ---- is just a volunteer.
For, to put it bluntly, the Sunday School teacher has other things in life that they also need to attend to and you get what you pay for.
If asked to do the other workaday work of the Sunday school teacher, it is doubtful the pastor could do that job without the book or operational manual either.
If these pastors want Sunday school teachers as absorbed in the nuances of Scripture and doctrine as professional clergy, pay the Sunday School teachers the wages of a pastor or staff member at a church that already has at least a half dozen pastors and compensated assistants already on the payroll.
By Frederick Meekins