"The Church doesn't err. Never has and never will. Bishops may err. Priests may err. Christian people may err. Whole dioceses may err. But the Church never can err."
Hold that thought.
"The problem with you Lutherans is that you have a Platonic notion of what the Church is."
Let's see: can anyone point to this "visible" church that cannot err? Oh, not that bishop! Oh, not his diocese! Oh, not this parish or that priest and certainly not that layperson!
I'm beginning to see that one cannot see.
It was a huge a-ha to me when I saw that Walther's true visible church was, well, invisible. It was just about as big of an a-ha when I realized that Orthodoxy's true visible church is invisible as well and in the exact same way.
What about Rome's? They sort of put all their "church cannot err" eggs into the papal basket, though they try to make it clear that it's not about the pope per se, but about the whole church, the infallibility given to the whole. But there sits Honorarius... Granted he didn't make his monothelite leanings an infallible pronouncement ex cathedra, but then again Rome didn't TALK that way then. But, wait a minute? Oh, never mind.
Luther's solution was rather simple. Worth reading too. You can find in AE 41:214 and following. His take? "Therefore the holy church cannot and may not lie or suffer false doctrine, but must teach nothing except what is holy and true, that is, God's Word alone; and where it teaches a lie it is idolatrous and the whore-church of the devil."
In other words, the Church by very definition is she who has and speaks the Word of God and it is that Word that *does not err, lie, or deceive.* When "church" presumes to speak what is NOT God's Word alongside God's Word, well, to the extent she does, she forfeits her claim of infallibility, because alongside of the inerrant Word she's mixed in stuff than can be quite fallible indeed.
All of which points us away from the sense of sight, to that of hearing, as Dr. Korby indicated long ago. Luther again: "The church must teach God's Word and truth alone, and not error or falsehood. How could it be otherwise? For God's mouth is the mouth of the church, and vice versa. God cannot lie, nor can the church." (AE 41:216) Find the Church in whose mouth is the infallible, inerrant Words of the living God and you have found the true Church. Oh, and as a bonus, she will be visible because she speaks through human mouths to human beings. But just remember her infallibility is not from this or that group that is speaking, but from what is spoken: the living oracles of the living God.