17 Yet let not the excessive and headlong faithlessness of many move or disturb us, but rather strengthen our faith in the truthfulness which has foretold the matter. As some have become such, because these things were predicted beforehand, so let other brethren beware of matters of a like kind, because these also were predicted beforehand, even as the Lord instructs us, and says, “But take heed: behold, I have told you all things.” Avoid, I beseech you, brethren, men of this kind, and drive away from your side and from your ears, as if it were the contagion of death, their mischievous conversation; as it is written, “Hedge your ears about with thorns, and refuse to hear a wicked tongue.” 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.
Source: The Treatises of Cyprian (New Advent)