Firmilian of Caesarea, Letter to Cyprian (Ep. 74/75) 24

Consider the rashness with which you dare to rebuke those who strive for the truth against falsehood. For who should more justly be indignant against another: the one who claims they are enemies of God, or the one who stands for the truth of the Church against the one who makes that claim? Unless it is simply that the rash are clearly also spirited and irascible, easily turning to anger through lack of counsel and speech, so that divine Scripture says of no one more than of you: "A spirited man prepares lawsuits, and an irascible man exaggerates sins" (Prov 17:19 LXX; cf. Prov 29:22). For how many lawsuits and dissensions have you stirred up throughout the churches of the whole world? How great a sin have you heaped upon yourself by cutting yourself off from so many flocks? You have cast yourself out. Do not deceive yourself; for the true schismatic is the one who makes himself an apostate from the communion of ecclesiastical unity: while you think that everyone else can be kept away from you, you alone have kept yourself away from everyone. Nor could the Apostle's precepts, warning and saying, instruct you according to the rule of truth and peace: "I therefore, a prisoner in the Lord, urge you to walk worthy of the calling with which you have been called, with all humility and gentleness, with patience, bearing with one another in love, eager to keep the unity of the Spirit in the bond of peace; one body and one Spirit, just as you were called in one hope of your calling; one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all" (Eph 4:1–6).