What was the old pastor preaching against in his denomination?
The old pastor was preaching against his own denomination's departure from the truth, which was already taking place in its higher echelons. The leadership began the slide, and every denomination can take that same track. That was the issue he was castigating.
Les explains that the old pastor was castigating his own denomination for departing from the truth, a departure already present in the higher echelons of the denomination. Les adds that this is nothing new: every denomination takes that track, and their leadership begins the slide. Aaron illustrates the same kind of failure—he should have stood in the gap while Moses was gone, but instead gave in to the demands of the people. Les also notes that many preachers are beholden to keeping everyone content because their salary depends on it, but he is not, so he can teach the book without compromise.