From On The Unity of the Church (251)
There is easy proof for faith in a short summary of the truth. The Lord speaks to Peter, saying, “I say unto thee, that thou art Peter; and upon this rock I will build my Church, and the gates of hell shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven; and whatsoever thou shalt bind on earth shall be bound also in heaven, and whatsoever thou shalt loose on earth shall be loosed in heaven” (Matt. 16:18-19). And again, to the same he says, after his resurrection, “Feed my sheep” (John 21:15).
And although to all the apostles, after his resurrection, he gives an equal power, and says, “As the Father hath sent me, even so send I you: Receive ye the Holy Ghost: Whose soever sins ye remit, they shall be remitted unto him; and whose soever sins ye retain, they shall be retained” (John 20:21); yet, that he might set forth unity, he arranged by his authority the origin of that unity, as beginning from one. Assuredly the rest of the apostles were also the same as Peter, endowed with a like partnership both of honor and power; but the beginning proceeds from unity. Which one Church, also, the Holy Spirit in the Song of Songs designated in the person of our Lord, and says, “My dove, my spotless one, is but one. She is the only one of her mother, elect of her that bare her” (Song 6:19).
Does he who does not hold this unity of the Church think that he holds the faith? Does he who strives against and resists the Church trust that he is in the Church, when moreover the blessed Apostle Paul teaches the same thing, and sets forth the sacrament of unity, saying, “There is one body and one spirit, one hope of your calling, one Lord, one faith, one baptism, one God” (Eph. 4:4).
And this unity we ought firmly to hold and assert, especially those of us that are bishops who preside in the Church, that we may also prove the episcopate itself to be one and undivided. Let no one deceive the brotherhood by a falsehood: let no one corrupt the truth of the faith by perfidious prevarication.
The episcopate is one, each part of which is held by each one for the whole. The Church also is one, which is spread abroad far and wide into a multitude by an increase of fruitfulness. As there are many rays of the sun, but one light; and many branches of a tree, but one strength based in its tenacious root; and since from one spring flow many streams, although the multiplicity seems diffused in the liberality of an overflowing abundance, yet the unity is still preserved in the source.
Separate a ray of the sun from its body of light, its unity does not allow a division of light; break a branch from a tree – when broken, it will not be able to bud; cut off the stream from its fountain, and that which is cut off dries up. Thus also the Church, shone over with the light of the Lord, sheds forth her rays over the whole world, yet it is one light which is everywhere diffused, nor is the unity of the body separated. Her fruitful abundance spreads her branches over the whole world. She broadly expands her rivers, liberally flowing, yet her head is one, her source one; and she is one mother, plentiful in the results of fruitfulness: from her womb we are born, by her milk we are nourished, by her spirit we are animated.
St. Cyprian of Carthage (ca. 200-258) was a gifted rhetorician who was converted to Christianity and became Bishop of Carthage. He led the church there during two waves of persecution, and defended the offer of reconciliation to those who had lapsed through fear of their lives. The treatise “On the Unity of the Church” is an attack on the rigorists led by Novatian, who had broken away to create “a church of the pure.” Cyprian was beheaded for the faith during a local wave of persecution and his feast is celebrated on several different days in mid-September by various churches.