"Brothers, be on your guard: the Devil is very crafty, but Christ is the wisdom of God."

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"frater, cauti estote; multum astutus est diabolus; sed Christus dei sapientia est."

     Augustine, Letter 64 to Quintianus, FC 12 (1951), trans. Sister Wilfrid Parsons, 312 =CSEL 34.1 (1895), ed. Al. Goldbacher, 232.  Why "frater" is in the singular, but "cauti" and "estote" are in the plural, I do not know.  But though CSEL 34.1 offers no variant readings, PL 33, col. 234 has "Fratres" (the less difficult reading).