Occasion of the discussion
Augustine is introducing the occasion: during the octave days, Christian laymen gathered with him and began discussing Christian religion against pagan claims, which led to the question about demons predicting the destruction of the temple of Serapis.
One day during the holy days of the octaves, when many lay Christian brothers had been with me in the morning and we had sat down in our usual place, a discussion arose about the Christian religion, set against the presumption and the supposedly wonderful and great knowledge of the pagans. I thought this discussion, once remembered and completed, should be committed to writing, without naming the persons who raised objections—though they were Christians and seemed to be asking, more by way of contradiction, what ought to be answered to pagans. So when the question of the divination of demons came up, and it was asserted that some person or other had predicted the overthrow of the temple of Serapis, which took place in Alexandria, I answered that it was not surprising if the demons were able both to know and to predict that this overthrow was threatening their temples and idols, just as they can know and foretell many other things, to the extent that they are permitted.
Latin
Quodam die in diebus sanctis octauarum, cum mane apud me fuissent multi fratres laici christiani et in loco solito consedissemus, ortus est sermo de religione christiana aduersus praesumptionem et tamquam miram et magnam scientiam paganorum: quem recordatum atque conpletum litteris mandandum putaui non expressis personis contradicentium, quamuis christiani essent et magis contradicendo quaerere uiderentur, quid paganis responderi oporteret. cum ergo de diuinatione daemonum quaereretur et affirmaretur praedixisse nescio quem euersionem templi Serapis, quae in Alexandria facta est, respondi non esse mirandum, si istam euersionem templis et simulacris suis inminere daemones et scire et praedicere potuerunt sicut alia multa, quantum eis nosse et praenuntiare permittitur.