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  • On the Holy Trinity [De Trinitate, Libri XV], by St. Augustin [Bishop St. Augustine of Hippo], translated by Arthur West Haddan, revised and annotated, with an introductory essay, by Professor W.G.T. Shedd. In A Select Library of the Nicene and Post-Nicene Fathers, edited by Dr. Philip Schaff, Volume III. St. Augustin: On the Holy Trinity; Doctrinal Treatises; Moral Treatises (Buffalo: Christian Literature Company, 1887): pp. 1-228. May be read online, and available in pdf, mobi, epub, and other formats at Internet Archive, and at Online Library of Liberty. Also available at Google Play Books
  • The text as republished in Nicene and Post-Nicene Fathers, First Series, Volume 3, edited by Philip Schaff (Buffalo, NY: Christian Literature Publishing Co., 1887) may be read online and is available in pdf, mobi, epub, and other formats at Christian Classics Ethereal Library. The text, sans preface and notes, may be read online on this page of Catholic Culture (copyright © 2021 Trinity Communications) and on this page of New Advent (copyright © 2020 by Kevin Knight).
  • The original translation is also available: On the Trinity, [by Bishop St. Augustine of Hippo,] translated by Arthur West Haddan. Volume VII of The Works of Aurelius Augustine, Bishop of Hippo: A New Translation, edited by Marcus Dods (Edinburgh: T.&T. Clark, 1873). May be read online, and available in pdf, mobi, epub, and other formats at Internet Archive, with a 2nd copy at Internet Archive. Also available at Google Play Books.

On the Feast of the Most Holy Trinity in the Roman Rite. For other texts and ebooks, you may access the List of Free eBooks (Arranged by Title), the List of Free eBooks (Grouped by Subject), the List of Worth-It Catholic Books & eBooks, and the main page of the Catholic eBooks Project. Blessed be the Holy and Undivided Trinity!

Going, therefore, teach ye all nations: baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” (St. Matthew xxviii, 19)

And I will ask the Father, and he shall give you another Paraclete, that he may abide with you for ever, The Spirit of truth… [H]e will teach you all truth… He shall glorify me: because he shall receive of mine, and will declare it to you. All things whatsoever the Father hath are mine. Therefore, I said, that he shall receive of mine, and will shew it to you.” (St. John xiv, 16-17, xvi, 13-15).