The four marks of the church, according to Catholicism, are found in the Nicene Creed, which says, “We believe in one holy catholic and apostolic church.” What follows are two statements concerning the first of these marks, one from the second century (representing Catholicism) and the other from the seventeenth century (representing Protestantism).
"For [the Church] is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? For how should it be if the apostles themselves had not left us writings? Would it not be necessary, [in that case,] to follow the course of the tradition which they handed down to those to whom they did commit the Churches? ... Now all these [heretics] are of much later date than the bishops to whom the apostles committed the Churches… since they are blind to the truth, and deviate from the [right] way, they will walk in various roads; and therefore the footsteps of their doctrine are scattered here and there without agreement or connection. But the path of those belonging to the Church circumscribes the whole world, as possessing the sure tradition from the apostles, and gives unto us to see that the faith of all is one and the same…." Irenaeus of Lyons, Against Heresies, Book 3, Chapter 4; Book 5, Chapter 20
"Among the attributes of the church, the first is unity, which flows from its nature. For since it is a holy society and a mystical body, embracing all the elect united in the bond of the same spirit, faith, and love with each other and with Christ, it must necessarily have a certain unity by which all its members may be mutually joined together…. Here we do not treat of its visible unity and as to its external state… we do not treat of an accidental unity either of place or time or rites or age… but we treat of an essential unity, which remains the same in various places as well as at various times, by which believers worship the same God, recognize the same Savior, have the same faith, are animated with the same spirit, and are members of the same body." Francis Turretin, Institutes of Elenctic Theology, Eighteenth Topic, Fifth Question, I-III.
Four points have been raised in the comments by Andrew McCallum concerning how we should read the early fathers. First, it is just as erroneous for the Catholic to insist that “If Protestants would just read the fathers they’d become Catholic” as it is for Protestants to insist that “If Catholics would just read the Bible they’d become Protestant.” The fathers must be read with an eye both to their philosophical and cultural milieu and their interpretive framework.
Secondly, we need to address the fact that, to the Protestant, it seems like the fathers are interpreted by Catholics anachronistically from the standpoint of contemporary Catholic dogma. In the light of the Catholic being bound by his Church’s interpretation of the fathers, how can reading them lead to anything but a fait accompli?
Thirdly, we need to discuss what exactly the fathers believed about their own authority. Did Clement or Polycarp believe they were writing with infallible authority? To the Protestant it appears that the Catholic Church’s insistence that the fathers, as interpreted by the Church, are infallible is a classic example of begging the question. Whether or not the fathers expected to be read as infallible is precisely the issue to be proven or disproven, not taken as an a priori.
Lastly, the Catholic must demonstrate that his position is in fact falsifiable. Is it possible that the Catholic claim to authority can turn out to be wrong? And if so, how would we know?
That said, please weigh in on these two representative statements about the church’s unity, keeping in mind these larger interpretive issues.