[Peter Seewald] Everything revolves again and again on this point: what must the church salvage from her tradition and what must she, if the need arises, discard? How is this question decided? Is there a list with two columns? On the right: always valid; on the left: capable of renewal?
[Joseph Cardinal Ratzinger] No, it's obviously not that simple. But there are various degrees of importance in the tradition. It was once customary in theology to speak of degrees of certitude, and that was not so wrong. Many say that we have to go back to that. The term hierarchy of truths does seem to point in this direction, namely, that not everything has the same weight, that there are, so to speak, essentials, for example, the great conciliar decisions or what is stated in the Creed. These things are the Way and as such are vital to the church's existence; they belong to her inner identity. And then there are ramifications that are connected with these essentials and certainly belong to the whole tree but that are not all of the same importance. The identity of the church has clear distinguishing marks, so that it is not rigid but the identity of something living, which remains true to itself in the midst of development. (The Essential Pope Benedict XVI [New York, NY: HarperCollins, 2003], 128-129)
A Legitimate Distinction
A distinction between essentials and non-essentials has, of course, been a staple of Protestant apologetic discourse. I'm not aware of any notable Catholic apologists who deny this distinction, but such a denial is common enough on discussion boards and blogs (a recent example may be found here here). If reasoning with those who deny the distinction seems fruitless, then perhaps the current Pope's articulation will be sufficient to convince such dissenters of its legitimacy.
The Whole Tree
The fact that we speak in such metaphors ("whole tree") when discussing essentials undergirds that the identification of these, being at its core an ontological project, is no simple task for any religious community. The learned Pope Benedict XVI acknowledges the complexity of the issue within the Roman Catholic community. And as experience shows (or will show, if you have yet to attempt it yourself), the task of absolutely delimiting these essentials is challenging; a precise periphery proves evasive. Catholic apologists should expect, then, to see some measure of difficulty on the part of Protestants to articulate a highly precise account of the essentials of Protestantism.
What should be acceptable to all is a general account. To extend Ratzinger's metaphor, we perceive the tree of our tradition and we know the trunk is essential to its identity. Let us also say that the boughs are essentials, but the twigs are non-essential. Any attempt to analyze the entire tree will encounter borderline cases where it is difficult to tell when a bough is a twig and when a twig is a bough. The existence of such borderline cases does not invalidate the clear identification of the trunk, nor of clear cases of boughs and twigs.
Each and every disagreement over the nature of the essentials does not count against the overall project of seeking essentials. It is not enough to simply note that one Protestant here disagrees with another Protestant there as to the exact list of essentials. This unreasonably bypasses the clear essentials that do unite Protestant groups; that one part of a project is difficult or unresolvable does not render the whole difficult or unresolvable.
Additionally, such an appeal is easily overturned with a similar comparison between what one Catholic here claims as the essentials and what one Catholic there otherwise claims as the essentials. If disagreement at the periphery disqualifies any and every attempt to identify essentials, then every religious community must be agnostic as to their nature.
What would be of significance is if Protestants could not even provide a general account. Here, of course, we run into that critical issue of defining our terms--just what is meant by Protestant will greatly affect the outcome of any potential critique along these lines. But I leave it to our theological opponents, should they so desire it, to fill out the rest of the argument.