Cyprian (c. 200 – 258), Bishop of Cathage, is quoted by John Calvin to prove that Protestants are neither Schismatics or Heretics. Cyprian uses a beautiful metaphor in his On the Unity of the Catholic Church (V) to explain the Oneness of the Church by comparing the Church's unity to the manifold rays of the one Sun, and to the many branches of one great tree, and also to the sundry streams that flow from one tributary. John Calvin employs this quote to demonstrate that where there is light, it is evident that the Sun has shined forth, and where there are buds on the branch, then it is evident that the tree is bearing fruit bearing, and where there is living water, there is a fount that has overflowed. The argument demonstrates that because the Protestant Church bears much light, fruit and living water, surely the Protestant Church is truly part of the One, Holy, and Apostolic Church, and may not have only done so otherwise. If the Protestant Church was not part of the one true Church, it would could do nothing:  I am the vine, you are the branches. Those who abide in me and I in them bear much fruit, because apart from me you can do nothing (John 15:5).  Light, Life and Living Water, what a Holy Trinity!

Calvin continuously writes against the Sin of Schism throughout the Institutes. It's a lesson that Protestants are desperately in need of learning today! Schism is more dangerous than heresy, in that it divides the body of Christ, where heresy is an infection of the body of Christ. Both schism and heresy threaten the health of the Church but it is Schism that causes the most danger. Calvin anticipates the critique that the Protestants are the Schismatics because they have departed from the One, Holy, Apostolic and Catholic Church (according to the Romanists - as Calvin calls them). Calvin responds to this accusation by quoting the bull of Leo X, in which Protestants were put out of the One Church by the Romanists. It's a brilliant response that demonstrates that one does not have to leave the Church to be a schismatic, one only needs to cause division of the body to be a schismatic. Calvin believes that the Romanists are the true schismatics because they have put out the Protestants from the fold! Calvin ingeniously equates Leo X's bull of excommunication with Jesus's prophecy that the apostles themselves would be put out of the synagogue (which the were indeed!).

The Protestant Church is shattered into a myriad shards like a dropped mirror, and so the dangers of schism should ever be before us! As Paul said, we look in the mirror dimly, because its shattered by schism. I've provided this quotation from Calvin's Institutes of the Christian Religion (IV.ii.6) to demonstrate the danger of schism, and to remind us that we do not need to leave the church to be schismatics! And that schism is as dangerous and more dangerous than the heretics we wish to put out from our synagogue.

Institutes IV.ii.6. Christ's headship the condition of unity*

Cyprian, also following Paul, derives the source of concord of the entire church from Christ's episcopate alone. Afterward he adds:

Cyprian (source: wikipedia)

"The church 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 strong trunk grounded in its tenacious root, and since from one spring flow many streams, although a goodly number seem outpoured from their bounty and superabundance, still, at the source unity abides. Take a ray from the body of the sun; its unity undergoes no division. Break a branch from a tree; the severed branch cannot sprout. Cut off a stream from its source; cut off, it dries up. So also the church, bathed in the light of the Lord, extends over the whole earth: yet there is one light diffused everywhere."⁽¹²⁾

Nothing more fitting could be said to express this indivisible connection which all members of Christ have with one another. We see how he continually calls us back to the Head himself. Accordingly, Cyprian declares that heresies and schisms arise because men return not to the Source of truth, seek not the Head, keep not the teaching of the Heavenly Master.

Now let them go and shout that we who have withdrawn from their church are heretics, since the sole cause of our separation is that they could in no way bear the pure profession of truth. I forbear to mention that they have expelled us with anathemas and curses⁽¹³⁾—more than sufficient reason to absolve us, unless they wish to condemn the apostles also as schismatics, whose case was like our own. Christ, I say, forewarned his apostles that they would be cast out of the synagogues for his name's sake [John 16:2]. Now those synagogues of which he speaks were then considered lawful churches. Since, therefore, it is clear that we have been cast out, and we are ready to show that this happened for Christ's sake, surely the case ought to be investigated before any decision is made about us, one way or the other. But I willingly grant them this point, if they so desire. For it is enough for me that it behooved us to withdraw from them that we might come to Christ.

Note 12: Cyprian, On the Unity of the Catholic Church v (MPL 4. 501 £.; CSEL 3. i. 213 f.; tr. ANF V. 423; LCC V. my)
Note 13 "Notorios et pertinaces haereticos . . . fuisse declarantes, eosdem . . . condemnamus." Leo X's bull Exsurge Domine (June 15, 1520); Mansi XXXII. 1051; Kidd, Documents, p. 79

Sources:

1. Calvin, John. Institutes of the Christian Religion. Ed. John T. McNeill. Trans. Ford Lewis. Battles. Philadelphia, PA: Westminster, 1960. 1024-025. Print.

2. Header Image Background: source: wikipedia