10 Hence heresies not only have frequently been originated, but continue to be so; while the perverted mind has no peace— while a discordant faithlessness does not maintain unity. But the Lord permits and suffers these things to be, while the choice of one's own liberty remains, so that while the discrimination of truth is testing our hearts and our minds, the sound faith of those that are approved may shine forth with manifest light. The Holy Spirit forewarns and says by the apostle, “It is needful also that there should be heresies, that they which are approved may be made manifest among you.” Thus the faithful are approved, thus the perfidious are detected; thus even here, before the day of judgment, the souls of the righteous and of the unrighteous are already divided, and the chaff is separated from the wheat.
These are they who of their own accord, without any divine arrangement, set themselves to preside among the daring strangers assembled, 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.
Source: The Treatises of Cyprian (New Advent)