Augustine of Hippo, Against Two Letters of the Pelagians II.2: The Heresies of the Manicheans and Pelagians are Mutually Opposed, and are Alike Reprobated by the Catholic Church

Chapter 2 [II.]—The Heresies of the Manicheans and Pelagians are Mutually Opposed, and are Alike Reprobated by the Catholic Church.

The Manicheans say that the good God is not the Creator of all natures; the Pelagians that God is not the Purifier, the Saviour, the Deliverer of all ages among men. The catholic Church condemns both; as well maintaining God’s creation against the Manicheans, that no nature may be denied to be framed by Him, as maintaining against the Pelagians that in all ages human nature must be sought after as ruined. The Manicheans rebuke the concupiscence of the flesh, not as if it were an accidental vice, but as if it were a nature bad from eternity; the Pelagians approve it as if it were no vice, but even a natural good. The catholic faith condemns both, saying to the Manicheans, “It is not nature, but it is vice;” saying to the Pelagians, “It is not of the Father, but it is of the world;” in order that both may allow it as an evil sickness to be cured—the former by ceasing to believe it, as it were, incurable, the latter by ceasing to proclaim it as laudable. The Manicheans deny that to a good man the beginning of evil came from free will; the Pelagians say that even a bad man has free will sufficiently to perform the good commandment. The catholic Church condemns both, saying to the former, “God made man upright,” and saying to the latter, “If the Son shall make you free, ye shall be free indeed.” The Manicheans say that the soul, as a particle of God, has sin by the commixture of an evil nature; the Pelagians say that the soul is upright, not indeed a particle, but a creature of God, and has not even in this corruptible life any sin. The catholic Church condemns both, saying to the Manicheans, “Either make the tree good and its fruit good, or make the tree evil and its fruit evil,” which would not be said to man who cannot make his own nature, unless because sin is not nature, but vice; and saying to the Pelagians, “If we say that we have no sin we deceive ourselves, and the truth is not in us.” In these diseases, opposed as they are to one another, the Manicheans and the Pelagians are at issue, with dissimilar will but with similar vanity, separated by different opinions, but close together by a perverse mind.