The 3 walls guarding the church

I'm currently re-reading Werner Elert's fine book Eucharist and Church Fellowship in the First Four Centuries. It's one of those books that bears re-reading every few years, first because it has depths that most of us are unable to plumb on one or 2 readings, not to mention that as we grow, our capacity for appreciation for a deep book grows.

The question I'm pondering now is regarding Elert's treatment of what he terms the "3 walls" guarding the integrity of the church in the first couple of centuries: the bishops, the canon of scripture (primarily the New Testament, as the Old Testament was a kind of given by that point), and the regula fidei, the creed of the church.

What I find intriguing is how Elert -- a Lutheran -- seems to stress the problems with the first wall, that of the bishops. He points out -- correctly -- that there were and always were erring bishops, and how this is not -- again, correctly -- a sure-fire means of guarding the church.

However, while he also points out the issues with the other 2 walls (for example, how heretics made their arguments buttressed with scriptural arguments) his stress seems to be on the problems with the bishops.

I'm wondering: was Elert correct? Was this as much of a problem as he seems to stress? Or is this a reflection of Lutheranism's sometimes problematic ecclesiology?