Demons' predictions are limited and often false
Augustine is contrasting the limited, often mistaken predictions of demons with the true prophecy that God works through angels and prophets.
By this and similar abilities demons announce many future things beforehand, yet the height of that prophecy which God works through his holy angels and prophets remains far beyond them. For if they announce anything from God's own disposition, they hear it so that they may announce it; and when they proclaim what they hear from there, they do not deceive and are not deceived, for the oracles of angels and prophets are utterly true. But some people take it indignantly that demons also hear and proclaim certain things of this kind, as if it were somehow unworthy that what is spoken precisely so that it may become known to human beings should not be kept silent by the wicked as well as the good. After all, even among human beings we see the precepts of a good life sung equally by the righteous and the perverse, and this does no harm; in fact it even helps the truth become better known and more widely famed when those who contradict it by their perverse conduct still say whatever they know about it. In their other predictions, however, demons are for the most part deceived and deceive. They are deceived because, when they announce their own dispositions, something is suddenly commanded from above that throws all their plans into confusion. It is as if certain people subject to various authorities were to arrange something they thought their superiors would not forbid, and promise they would carry it out—and then those who hold greater power, acting on some higher counsel, suddenly forbid the whole thing that had been arranged and prepared. They are also deceived in natural causes—just as physicians, sailors, and farmers are, though the demons foresee far more keenly and far more excellently according to the more skilled and more practiced sense of an airy body—because these things too are suddenly and unexpectedly changed by angels who serve the supreme God in piety, acting from another disposition unknown to demons. It is as if something should happen to a sick man from outside that kills him, after a physician had promised on the basis of true preceding signs of health that he would live; or as if some sailors had predicted from the observed quality of the air that a certain wind would blow for a long time, and then the Lord Christ, sailing with his disciples, commanded it to be still, and a great calm came about (Matthew 8:26); or as if a farmer, skilled in the nature of the soil and the number of seeds, should promise that a certain vine would bear fruit that year, and then either an unexpected harshness of the sky should dry it up or some command of a higher power should uproot it. In the same way, many things that pertain to the foreknowledge and prediction of demons—things foreseen as future from lesser and more ordinary causes—are impeded and changed by greater and more hidden causes. They also deceive by zeal for deceiving and by an envious will, through which they rejoice in human error. But so that they do not lose the weight of authority among their worshipers, they make sure that the blame is assigned to their interpreters and the conjecturers of their signs whenever they have been deceived or have lied.
Latin
Hac atque huius modi facultate multa daemones futura praenuntiant, cum tamen ab eis longe sit altitudo illius prophetiae, quam deus per sanctos angelos et prophetas operatur. nam si quid de illa dei dispositione praenuntiant, audiunt. ut praenuntient; et cum ea praedicunt, quae inde audiunt, non fallunt neque falluntur; ueracissima enim sunt angelica et prophetica oracula. sic autem indignanter accipitur, quod aliqua etiam talia daemones audiant et praedicant, quasi aliquid indignum sit, ut quod ideo dicitur, ut hominibus innotescat, hoc non solum boni, uerum etiam mali non taceant. cum in ipsis hominibus etiam uitae bonae praecepta uideamus pariter a iustis peruersisque cantari, non obesse aliquid, immo etiam prodesse ad maiorem notitiam famamque ueritatis, cum de illa etiam hi quidquid nouerunt dicunt, qui ei peruersis moribus contradicunt. in ceteris autem suis praedicationibus daemones plerumque falluntur et fallunt. falluntur quidem, quia, cum suas dispositiones praenuntiant, ex inprouiso de- . super aliquid iubetur, quod eorum consilia cuncta perturbet. uelut si aliquid disponant quilibet homines quibusque potestatibus subditi, quod non prohibituros suos praepositos arbitrentur idque facturos se esse promittant, at illi, penes quos maior potestas est, ex alio superiore consilio hoc totum dispositum atque praeparatum repente prohibeant. falluntur etiam in causis naturalibus — sicut medici et nautae et agricolae, sed longe acutius longeque praestantius pro aerii corporis sensu sollertiore et exercitatiore praenoscunt — quia et haec ab angelis deo summo pie seruientibus ex alia dispositione ignota daemonibus ex inprouiso ac repente mutantur. tamquam si aliquid aegro extrinsecus accidat, quo moriatur, quem medicus ueris salutis praecedentibus signis uicturum esse promiserat; aut si aeris qualitate praeuisa diu flaturum illum uentum nautarum aliqui praedixissent, cui dominus Christus cum discipulis nauigans, ut quiesceret, imperauit, et facta est tranquillitas magna; uel si agricola illo anno fructificaturam uitem aliquam polliceatur, naturam utique terrae et numerum seminum callens, quo tamen anno eam uel inprouisa caeli intemperies arefaciat uel aliqua iussio potentioris eradicet: ita multa etiam ad praescientiam praedictionemque daemonum pertinentia, quae minoribus et usitatioribus causis futura praeuidentur, eadem maioribus et occultioribus causis inpedita mutantur. fallunt autem etiam studio fallendi et inuida uoluntate, qua hominum errore laetantur. sed ne apud cultores suos pondus auctoritatis amittant, id agunt, ut interpretibus suis signorumque suorum coniectoribus culpa tribuatur, quando uel decepti fuerint uel mentiti.