St. Cyprian of Carthage, Treatise I:
"who appoint themselves prelates without any law of ordination, who assume to themselves the name of bishop, although no one gives them the episcopate; whom the Holy Spirit points out in the Psalms as sitting in the seat of pestilence, plagues, and spots of the faith, deceiving with serpent's tongue, and artful in corrupting the truth, vomiting forth deadly poisons from pestilential tongues; whose speech does creep like a cancer, whose discourse forms a deadly poison in the heart and breast of every one.
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.
This evil, most faithful brethren, had long ago begun, but now the mischievous destruction of the same evil has increased, and the envenomed plague of heretical perversity and schisms has begun to spring forth and shoot anew; because even thus it must be in the decline of the world, since the Holy Spirit foretells and forewarns us by the apostle, saying, "In the last days," says he, "perilous times shall come, and men shall be lovers of their own selves, proud, boasters, covetous, blasphemers, disobedient to parents, unthankful, unholy, without natural affection, truce-breakers, false accusers, incontinent, fierce, hating the good, traitors, heady, high-minded, lovers of pleasures more than lovers of God, having a sort of form of religion, but denying the power thereof.
And again, "Evil communications corrupt good manners." The Lord teaches and warns us to depart from such. He says, "They are blind leaders of the blind; and if the blind lead the blind, they shall both fall into the ditch." Such a one is to be turned away from and avoided, whosoever he may be, that is separated from the Church. Such a one is perverted and sins, and is condemned of his own self. Does he think that he has Christ, who acts in opposition to Christ's priests, who separates himself from the company of His clergy and people? He bears arms against the Church, he contends against God's appointment. An enemy of the altar, a rebel against Christ's sacrifice, for the faith faithless, for religion profane, a disobedient servant, an impious son, a hostile brother, despising the bishops, and forsaking God's priests, he dares to set up another altar, to make another prayer with unauthorized words, to profane the truth of the Lord's offering by false sacrifices, and not to know that he who strives against the appointment of God, is punished on account of the daring of his temerity by divine visitation.
These, doubtless, they imitate and follow, who, despising God's tradition, seek after strange doctrines, and bring in teachings of human appointment, whom the Lord rebukes and reproves in His Gospel, saying, "You reject the commandment of God, that you may keep your own tradition." This is a worse crime than that which the lapsed seem to have fallen into, who nevertheless, standing as penitents for their crime... puffed up in his heart, and pleasing himself in his very crimes, separates sons from their Mother, entices sheep from their shepherd, disturbs the sacraments of God; and while the lapsed has sinned but once, he sins daily. Finally, the lapsed, who has subsequently attained to martyrdom, may receive the promises of the kingdom; while the other, if he have been slain without the Church, cannot attain to the rewards of the Church.