Some quick math (quantatative analysis)

List of A/PNF (and non-fathers) at New Advent

• Less than a dozen in the list are primarily addressed to heathens / specific philosophic communities
• Two (three counting Novatian's letter on meat) to Jews
• Many on "Christian Living" (Marriage, Widows, Modesty, Fasting)
• Many commentaries/sermons on scripture
• Many against Arians and Gnostics (heretics)
• Many histories of the Saints

Conclusion: Heretics a much larger concern than Pagans/Heathens

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True, St. Augustine, the Doctor of Grace's, book de civitate dei contra paganos (City of God, Against the Pagans) is the longest book in the list, seemingly as long as the rest of the list put together... so by page count this may be completely inverted