St. Clement of Rome (96AD):
The Apostles received the gospel for us from the Lord Jesus Christ: Jesus the Christ was sent from God. Thus Christ is from God, the Apostles from Christ: in both cases the process was orderly, and derived from the will of God….They [the Apostles] preached in country and town, and appointed their first fruits, after testing them by the Spirit, to be bishops and deacons of those who were going to believe. And this was no novelty….Our Apostles also knew, through our Lord Jesus Christ, that there would be strife on the question of the bishop’s office. Therefore…they appointed the aforesaid persons and later made further provision that if they should fall asleep, other tested men should succeed to their ministry. (First Epistle to the Corinthians, xlii-xliv)
The apostles, then, in like manner founded churches in every city, from which all the other churches—one after another—borrowed the tradition of the faith and the seeds of doctrine. And they are every day borrowing them, that they may become churches. Indeed, it is only on this account that they will be able to deem themselves apostolic—as being the offspring of apostolic churches….Therefore the churches, although they are so many and so great, comprise but the one primitive church of the apostles—from which they all [spring].
We hold communion with the apostolic churches because our doctrine is in no respect different than theirs. This is our witness of truth. (3.252, 253).
St. Irenaeus of Lyons (202AD):
By “knowledge of the truth” we mean: the teaching of the Apostles; the order of the Church as established from the earliest times throughout the world: the distinctive stamp of the Body of Christ, preserved through the Episcopal succession: for to the bishops the Apostles committed the care of the church which is in each place, which has come down to our own time, safeguarded without any written documents…. (Against Heresies, iv. xxxiii. 8)
The catholic Church possesses one and the same faith throughout the whole world, as we have already said (Against Heresies 1:10 [A.D. 189]).
St. Cyprian of Carthage (200-258):
“The episcopate is one….The Church is one….So also the Church, flooded with the light of the Lord, extends her rays over all the globe: yet it is one light which is diffused everywhere and the unity of the body is not broken up.” (On the Unity of the Catholic Church)
“This sacrament of unity [the Church], this bond of peace inseparable and indivisible, is indicated when in the Gospel the robe of the Lord Jesus Christ was not divided at all or rent, but they cast lots for the raiment…so the raiment was received whole and the robe was taken unspoilt and undivided.”
“…and the Church is made up of the people united to their priest, the flock cleaving to its shepherd. Hence you should know that the bishop is in the Church, and the Church in the bishop, and that if anyone is not with the bishop he is not in the Church…the Church is catholic and one, and may not be sundered or divided but should assuredly be kept together and united by the glue which is the mutual adherence of the priest.”
“Some of our colleagues, by a curious presumption, are led to suppose that those who have been dipped [baptized] among the heretics ought not to be baptized when they join us; because, they say, there is ‘one baptism’. Yes, but that one baptism is in the catholic Church. And if there is one Church, there can be no baptism outside it….Our assertion is that those who come to us from heresy are baptized by us, not re-baptized.”