Do not let your deeds belie your words, lest when you speak in church someone may say to himself, “Why do you not practice what you preach?”

St. Jerome (c. 347-419) Roman Christian priest, theologian, historian, translator [Eusebius Sophronius Hieronymus]

Letter 52, to Nepotian (AD 394)

Alt. trans.: "Do not let your deeds belie your words; lest when you speak in church someone may mentally reply, 'Why do you not practice what you profess?'" [Philip Schaff & Henry Wace (eds.) A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, vol. 6 (1893)]