Ante-Nicene Fathers Volume 5
Cyprian: Epistle LIV
Apparently, some of the heretics who had been excommunicated for various immoralities and for schism had been threatening the church in Rome. In the lengthy Epistle LIV, Cyprian encourages Cornelius to stand firm in the faith and continue to do his duty. This not because we are awesome, but because of the Scriptural promises God has made to His church concerning its authority and ultimate triumph. And while not every example Cyprian uses is necessarily an appropriate application of Scripture, his overall point remains a good one. There is no heresy or assault on the church that God has not provided for, and we need never fail to do our duty because of the sin of unbelievers. Paragraph 5 especially gives us Cyprian's view of the unity of the local church, "For neither have heresies arisen, nor have schisms originated, from any other source than from this, that God's priest is not obeyed."
This is true, so long as we add the caveat "God's priest is not obeyed when he is preaching the true Gospel." Fortunately, this is in fact Cyprian's concern. We do not want to let unbelievers (even if they are claiming to be Christians) dictate the life and teachings of the church: "Is the dignity of the catholic church,dearest brother, to be laid aside... for this, that those who are set without the church may say that they wish to judge concerning a prelate in the church? heretics concerning a Christian? wounded men about a whole man? maimed concerning a sound man?" And so on. While his view of the church can at times be skewed, Cyprian's goals are exactly what they should be.
With that said, this letter is long and can be skimmed, other than paragraphs 5,6, and 18.