Augustine of Hippo

Variously titled, “Psalm Against the Party of the Donatists,” “Alphabetical Psalm Against the Donatists,” “One Book, a Psalm against the Party of the Donatists,” [2] etc. Latin text in Migne Patrologia Latina (PL) 43:
23-32 [3] "St. Augustine began his victorious campaign against Donatism soon after he was ordained priest in 391. His popular psalm or "Abecedarium" against the Donatists was intended to make known to the people the arguments set forth by St. Optatus, with the same conciliatory end in view. It shows that the sect was founded by traditors, condemned by pope and council, separated from the whole world, a cause of division, violence, and bloodshed;* the true Church is the one Vine, whose branches are over all the earth." - Catholic Encyclopedia [4] * Augustine frequently complained of the Donatist's violence against the Catholics (see e.g. letters 105, 185 209). Even so, he maintained a deep pastoral love and concern for them, [5] and ever strove for their eventual return to "the Unity," one of his "favorite names for the Catholic Church." [6].

See also Confessions

De Civitate Dei full text online at Wikisource as translated by Rev. George Wilson and Rev. J. J. Smith

Quotations are from Wilson and Smith translation unless otherwise specified.

Quid, si falleris? Si enim fallor, sum. Nam qui non est, utique nec falli potest; ac per hoc sum, si fallor. Quia ergo sum si fallor, quo modo esse me fallor, quando certum est me esse, si fallor.

What difference, if you are mistaken? For if I am mistaken, I am. For he who is not, assuredly cannot be mistaken; and therefore I am, if I am mistaken. Therefore because I am if I am mistaken, how am I mistaken that I am, when it is sure that I am, if I am mistaken.'

The Latin variations of the statement sum si fallor (I am because I err), have sometimes become paraphrased Fallor, ergo sum (I err, therefore I am), based on the form of the later Cogito, ergo sum (I think, therefore I am) of Principles of Philosophy (1644) by Descartes. Familiarity with Augustine's thought could have actually inspired some of Descartes statements here and in the earlier Meditations on First Philosophy (1641):
"Doubt is the origin of wisdom.".

Ad Catholicos Epistula contra Donatistas (Letter to the Catholics against the Donatists), or De Unitate Ecclesiae (On the Unity of the Church), [10] Ad Catholicos epistola contra Donatistas vulgo De Unitate Ecclesiae liber unus. Latin text in Migne, Patrologia Latina (PL), 43:
391–446. [11] Variant: Epistula ad Catholicos de secta Donatistarum.

See also:
First Epistle of John

De trinitate full Latin text at Wikisource

Quotes reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895).

From The Whole Christ:
The Historical Development of the Doctrine of the Mystical Body in Scripture and Tradition (1938, 1962), Fr. Emile Mersch, S. J. translated by, John R. Kelly, S.J, Part 3. The Doctrine of the Mystical Body in Western Tradition, Ch. 4, Augustine’s Sermons to the People

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