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Which writings by St. Augustine have had the greatest influence?

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Justus of Ghent: Saint Augustine — St. Augustine *Saint Augustine*, oil on wood panel by Justus of Ghent, c. 1475; in the Louvre Museum, Paris.

St. Augustine of Hippo was a prolific author, leaving behind over 100 books and 500 sermons that helped establish the foundation for medieval and modern Christian thought. While his body of work is vast, two writings are considered to have had the most profound and lasting influence:12

Confessions: Written around 400 CE, this work is a spiritual self-examination that explores the philosophical and emotional development of the human soul. It broke new ground in literature, effectively creating the genre of autobiography, and provides deep psychological insights into memory, knowledge, and love.134
The City of God: Composed between 413 and 426 CE, this philosophical treatise was written to defend Christianity following the sack of Rome. It outlines a vision of two competing societies—the "City of God" and the "City of Man"—and offers a linear, predestined interpretation of human history. It served as a model for church-state relations for centuries and influenced major theologians like St. Thomas Aquinas and John Calvin.15

Additionally, his work De trinitate (The Trinity) remains a significant theological contribution, exploring the nature of the Trinity through the lens of human psychology.64