It seems a circular reasoning to say that the Church has never erred and if she has, the error was not hers but one of her erring servants. It seems another circular reasoning to say that the Pope has not erred and if he has, he was not speaking from the Chair of St. Peter infallibly. Well, of course that is how it must be. The reality is that the Church has suffered through many erring teachers and preachers and popes and councils and magisteria. Scripture cannot lie or deceive but even Scripture in the hands of a false teacher can be twisted by error. This infallibility cannot be deposited in flesh and blood or even an institution but Scripture -- not a naked Scripture as some would suggest but a Scripture that lives within the community of faith, attested to and attesting to the living tradition that has and will always surround that Word of God, and within the witness of creed and confession.
The explicit teaching of the Church's infallibility was not brought forward until the first Vatican Council in direct response, it would seem, to the contention of the Reformation and its heirs that the Church did err in raising what is not explicitly in Scripture to the level of Scripture and assigning these doctrines to be essential. Of course, Christ established the Church and bestowed upon the Church His own teaching authority along with the promise of the Spirit. This was not simply to the apostles in their lifetime but the Church until the end of times. Yet with this promise did not come the assurance that error was impossible. Just the opposite, warning was given explicitly in Scripture to hold fast to the doctrine once delivered and not to add or take away even a word from Scripture. Everywhere there was and is the promise of an unchanging truth, it is posited in the Scripture and only by extension to those who keep and proclaim this Scripture faithfully.
Unity of faith was manifested to the Church not by virtue of a person or office but by the Spirit working through the Word of God. It is not a Word subject to private interpretation or hidden meaning that must be explicated by the reason or erudition of man. It is the work of the Spirit to impart knowledge and understanding -- and more importantly faith. The promise that the gates of hell shall not prevail is not a promise that the Church shall not ever have to suffer error but that the devil cannot win what Christ has already won. If error does creep into the Church, her appeal is not to an institution or an office but to the Word of God. This Word is a reforming and renewing Word for the Church as well as a Word of witness to those not yet of the Kingdom.
Even Luther would admit (though perhaps none of the radical reformers) that even in the darkest hours of the history of the Church the Word remained to bring forth faith in the hearer and the Sacraments were efficacious in supplying the mercies and merits of Christ and His promises to the people of God. If this is what infallibility means, then we could grant it gladly but that is not what Rome means. Rome insists that the magisterium preserves the Church from all error in matters of faith and morals through the exercise of the papal office. Although individual bishops may err in their official teachings about faith and morals, the Magisterium as a whole never does, nor does the Pope. This relates not simply to the extraordinary teaching of the papal office but also to its ordinary teaching. But look at the current pope and the conundrum he offers Rome. He has overtly and by skirting on the edge of truth denied what his predecessors have said and what Scripture says, even gone so far as to say that a worship form used for more than four centuries is now abrogated by one that is still being refined. In short, Francis has done a pretty good job of proving by experience that the kind of infallibility of the Church, apart from Scripture, is a tenuous unproven and unprovable article of faith which even Scripture does not attest. Far better than struggling to maintain the untenable with regard to the institution and an office is to meet the promise of our Lord right where it is given in the Word of God.
When the Spirit of truth comes, he will guide you into all the truth,
for he will not speak on his own authority, but whatever he hears he
will speak, and he will declare to you the things that are to come. John 16:13 I have given them Your Word...Sanctify them in the truth; Your Word is truth. John 17