We are fairly certain today that, while the Fathers were not Roman Catholic as the thirteenth or nineteenth century would have understood the term, they were, nonetheless, "Catholic", and their Catholicism extended to the very canon of the New Testament itself.- Benedict XVI (then Joseph Ratzinger), Principles of Catholic theology: building stones for a fundamental theology (2:1:D), p. 141 (English edition, 1987 - Originally published in German in 1982)(see more context here)
Pastor King has said:
We, as Protestants, are very content to let the ECFs be what they were. But it is the Roman apologist who, on the contrary, must read back into the ECFs the notions of modern day Rome and papal primacy that were never recognized by the eastern church. Again, for all this insistence on the ECFs being “catholic” I am in great agreement!(source)
What is also interesting is that Ratzinger's comment stands opposed to lay Roman apologists who claim things like "The Church Fathers Were Catholic" (meaning, of course, "Roman Catholic") (Dave Armstrong, who has a book by that title, comes to mind, though he is not alone in making this sort of ignorant assertion).
Ratzinger goes on, of course, to insist that "only one side can consider them its own Fathers" but the admission that Ratzinger has made exposes one of the central weaknesses to much of the patristically-directed Roman apologetic effort in the English-speaking world today. We can agree with Ratzinger that the Fathers were "catholic" as that term is properly understood, and we can also agree with him that they would not be considered "Roman Catholic" by modern (or even medieval) standards. We too willingly acknowledge that the Fathers were not distinctly "Protestant" - they were who they were, often differing in significant ways from one another. As Pastor King explained it, we "are very content to let the [early church fathers] be what they were."
These facts ought, however, to point us to the need for an even earlier source of authority - the written Word of God contained in the Scriptures of the Old and New Testaments. By such an established authority we can evaluate the claims of apostolicity of the various competing claimants to the catholic and apostolic faith.