This is a question that is primarily for Catholics who dialogue with Protestants.
For many years I have been aware of Protestant religions that realize the early Church called itself Catholic, so they appropriate the term for themselves & instruct their followers to refer to themselves as “Catholic”, thereby denying the exclusive use of that term to what they call the “Roman” Catholic Church-- as if there were a significant difference between the ancient Catholic Church & the modern Catholic Church. They do this in order (1) to create the illusion that the ancient Catholic faith & belief system are much more directly related to their own than is the case, & (2) to bolster their claim that the ancient Catholic Church was a sort of generic “mother-Church” to all Protestant faiths, although they disparage any close relationship between the ancient Catholic Church & the modern Catholic Church. The splitting-point between the ancient Catholic Church & the modern Catholic Church is usually given as c. 330 AD & is associated with the reign of Constantine, although I recently encountered a poster on another thread who put the date as late as c. 600-800 AD. Unfortunately the poster got banned before I could explore his reasoning.
Following some version of the above reasoning is virtually the only way for a Protestant to get around very early references in Ignatius, Polycarp, Clement, & elsewhere to universal papal jurisdiction. Many traditional Baptists, as many of you are probably aware, even deny that they are Protestants at all, claiming that their religion dates back to the time of the Apostles.
I wonder about the best way to approach such people, in order to demonstrate the unity of the ancient Catholic Church with the modern Catholic Church, & that no Protestant sect is found anywhere prior to the 16th Century. I believe that their insistence on the above points indicates a deep-seated uncertainty about the true antiquity of their religion, & in some cases even a possible suspicion that the ancient Catholic Church & modern Catholic Church might be identical. If so, then it behooves us to help them towards that realization.
To show that this is possible, I have cited on another thread the experience of David J. Webster, a former fundamentalist Baptist minister who converted after finding what he describes as “…over 40 clear references to distinctly Catholic tenets of faith in the 135-year period immediately following the New Testament (AD 100-235).” (And my Protestant friends, before you swarm me claiming that “the ‘Roman’ Catholic Church didn’t exist at the time in question”, the statement isn’t mine-- it’s Webster’s. Although, for the sake of disambiguation, I do believe he’s correct & that the writings of the ECF strongly support his view.)
I will parenthetically observe that I personally don’t refer to my Church as ‘Roman’ Catholic, since I believe that to do so is to concede this point to Protestants who have unjustifiably appropriated the term for themselves. My Church is the only Catholic Church that has ever existed, exclusive of their unfounded claims, identical with the Church of the Apostles.
One approach would be doctrinal-- to take a sampling of 3 or 4 Catholic & Protestant doctrines, seek those doctrines in the writings of the ECF, & follow them thru to a few hundred years after the claimed splitting-point. This approach would be very time consuming, & I’m not sure how effective.
Another approach-- the one that I’ve usually favored myself-- is to demonstrate universal papal jurisdiction from an unimpeachable source at a very early date. This is an all-or-nothing approach, & the Protestant knows that if he concedes it then he’s lost his entire argument. Also, it’s a frontal attack, so Protestants are usually deep in their defensive trenches on this one.
Another approach that I’ve occasionally used is to show that the Roman pontiff, along with the college of Catholic bishops, was the universal arbiter of issues like the Easter controversy, the promulgation of the decrees of ecumenical councils, & the codification of the Bible from the 2d thru the 5th Centuries, & that other non-Catholic religions were nowhere involved. This approach is a little less in-your-face than the “universal jurisdiction” approach, but it often still gets hung up on side issues.
Obviously there’s no point in trying to prove anything to someone who is in bad faith to begin with, but for the benefit of someone who is genuinely seeking the truth I think we have an obligation to provide it in as convincing a manner as possible.