Around 401, while visiting Carthage, Augustine was asked to defend the conversion of a banker named Faustinus who, locals suspected, had his eye on the job of mayor and who realized he would never get it unless he converted to Christianity. Augustine argued to the wary Carthaginians that Faustinus be given the benefit of the doubt: “We cannot see into the human heart nor bring it out into the open ...” (s. 279.10). This scene and this claim illustrates a pervasive but under-appreciated theme in Augustine’s theology: our mutual indecipherability, our fundamental inaccessibility to one another’s true thoughts and feelings. This theme spans the length of his career and the breadth of his writings and undergirds many of his core theological concerns: his theories of language and of biblical interpretation (De doctrina christiana), his interior spirituality (Confessiones), his views on the Trinity (De fide et symbolo) and the resurrection (De civitate Dei). This essay will explore the theme’s diverse theological uses and implications, as well as its grounding in the practical experience of Augustine the orator.