He put fornication at the head of the carnal vices and love at the head of the spiritual virtues. Anyone who takes pains in the study of divine Scripture will be prompted will be prompted to inquire attentively to the rest. Fornication is love divorced from legitimate wedlock. It roves everywhere in search of an opportunity to fulfill its lust.Yet nothing is so rightly suited for spiritual procreation as the union of the Soul with God. The more firmly it adheres, the more blameless it is. Love is what enables it to cleave. Rightly then the opposite of fornication is love. It is he sole means by which chastity is observed (Galatians 85).
I haven't come across any other commentators so far (I've only looked at Longenecker, Dunn, and Martyn) discussing if there's a possible link between the heads of each of Paul's lists in Galatians 5, and I'm curious if there is. I've been wondering why the early church chose agape as the word to use to describe love when the verb form, prior to the New Testament, was used to describe sex. Is there any sense in which Paul is depending on that meaning here? Did the word simply change meanings and the old meaning disappear? Does any sexual overtone survive in other NT uses? I have no idea, but Augustine's quote is interesting to think about.