Saint Cyprian of Carthage wrote an entire treastise on the Unity of the Church, because even early on, there were attempts by the enemy to divide the faithful. The unity of the Church was so important to the early Christians that the bishops incorporated the concept in the early creeds. "I believe in One , Holy , Catholic and Apostolic Church."
Note Church, not churches. The unity of one shepherd, one flock was integral to the faith since the beginning. In Corinth, Saint Paul rebuked the faithful for attempting to denominate as Dr. Kreeft points out in my earlier post.
"Nor let any deceive themselves by a futile interpretation, in respect of the Lord having said, Wheresoever two or three are gathered together in my name, there am I in the midst of them. Matthew 18:20 Corrupters and false interpreters of the Gospel quote the last words, and lay aside the former ones, remembering part, and craftily suppressing part: as they themselves are separated from the Church, so they cut off the substance of one section
. For the Lord, when He would urge unanimity and peace upon His disciples, said, I say unto you, That if two of you shall agree on earth touching anything that you shall ask, it shall be given you by my Father which is in heaven. For wheresoever two or three are gathered together in my name, I am with them; showing that most is given, not to the multitude, but to the unanimity of those that pray. If, He says, two of you shall agree on earth: He placed agreement first; He has made the concord of peace a prerequisite; He taught that we should agree firmly and faithfully. But how can he agree with any one who does not agree with the booty of the Church itself, and with the universal brotherhood? How can two or three be assembled together in Christ's name, who, it is evident, are separated from Christ and from His Gospel? For we have not withdrawn from them, but they from us; and since heresies and schisms have risen subsequently, from their establishment for themselves of diverse places of worship, they have forsaken the Head and Source of the truth. "
" Who, then, is so wicked and faithless, who is so insane with the madness of discord, that either he should believe that the unity of God can be divided, or should dare to rend it— the garment of the Lord— the Church of Christ?
He Himself in His Gospel warns us, and teaches, saying, And there shall be one flock and one shepherd. John 10:16 And does any one believe that in one place there can be either many shepherds or many flocks? The Apostle Paul, moreover, urging upon us this same unity, beseeches and exhorts, saving, I beseech you, brethren, by the name of our Lord Jesus Christ, that you all speak the same thing, and that there be no schisms among you; but that you be joined together in the same mind and in the same judgment. 1 Corinthians 1:10 And again, he says, Forbearing one another in love, endeavouring to keep the unity of the Spirit in the bond of peace. Ephesians 4:3 Do you think that you can stand and live if you withdraw from the Church, building for yourself other homes and a different dwelling, when it is said to Rahab, in whom was prefigured the Church, Your father, and your mother, and your brethren, and all the house of your father, you shall gather unto you into your house; and it shall come to pass, whosoever shall go abroad beyond the door of your house, his blood shall be upon his own head?"