3 But, beloved brethren, not only must we beware of what is open and manifest, but also of what deceives by the craft of subtle fraud. And what can be more crafty, or what more subtle, than for this enemy, detected and cast down by the advent of Christ, after light has come to the nations, and saving rays have shone for the preservation of men, that the deaf might receive the hearing of spiritual grace, the blind might open their eyes to God, the weak might grow strong again with eternal health, the lame might run to the church, the dumb might pray with clear voices and prayers— seeing his idols forsaken, and his lanes and his temples deserted by the numerous concourse of believers— to devise a new fraud, and under the very title of the Christian name to deceive the incautious?
He has invented heresies and schisms, whereby he might subvert the faith, might corrupt the truth, might divide the unity. Those whom he cannot keep in the darkness of the old way, he circumvents and deceives by the error of a new way. He snatches men from the Church itself; and while they seem to themselves to have already approached to the light, and to have escaped the night of the world, he pours over them again, in their unconsciousness, new darkness; so that, although they do not stand firm with the Gospel of Christ, and with the observation and law of Christ, they still call themselves Christians, and, walking in darkness, they think that they have the light, while the adversary is flattering and deceiving, who, according to the apostle's word, transforms himself into an angel of light, and equips his ministers as if they were the ministers of righteousness, who maintain night instead of day, death for salvation, despair under the offer of hope, perfidy under the pretext of faith, antichrist under the name of Christ; so that, while they feign things like the truth, they make void the truth by their subtlety. This happens, beloved brethren, so long as we do not return to the source of truth, as we do not seek the head nor keep the teaching of the heavenly Master.
Source: The Treatises of Cyprian (New Advent)