Laudator Temporis Acti

Rodulfus Glaber, i.e. Ralph the Bald (985–1047), Historiarum libri quinque ab anno incarnationis DCCCC usque ad annum MXLIV, book 2, chapter 12, in Patrologia Latina 142, cols. 611-698 (at 644 A-C), tr. at Fordham University, Medieval Sourcebook: Ralph the Bald (ca.1025): Early Appearances of Heresy, c. 970. Vilgard at Ravenna, and Other Disturbances (http://www.fordham.edu/halsall/source/1025ralph-heresy.asp):

At that time also, mischief not unlike the above appeared at Ravenna. A certain man named Vilgard occupied himself with more eagerness than constancy in literary studies, for it was always the Italian habit to pursue these to the neglect of the other arts. Then one night when, puffed up with pride in the knowledge of his art, he had begun to reveal himself to be more stupid than wise, demons in the likeness of the poets Vergil, Horace, and Juvenal appeared to him, pretending thanks for the loving study which he devoted to the contents of their books and for serving as their happy herald to posterity. They promised him, moreover, that he would soon share their renown. Corrupted by these devilish deceptions, he began pompously to teach many things contrary to holy faith and made the assertion that the words of the poets deserved belief in all instances. But he was at last discovered to be a heretic and was condemned by Peter, archbishop of that city.

Many others holding this noxious doctrine were discovered throughout Italy, and they too died by sword and pyre. Indeed, at this same period some went forth from the island of Sardinia—which usually teems with this sort of folk—to infect the people of Spain, but they were exterminated by the Catholics. This accords with the prophecy of the apostle John, in which he said that Satan would be released when a thousand years has passed. Of this we shall treat more fully in a third book.

The Latin, from Patrologia Latina:

Ipso quoque tempore non impar apud Ravennam exortum est malum. Quidam igitur Vilgardus dictus, studio artis grammaticae magis assiduus quam frequens, sicut Italis mos semper fuit artes negligere caeteras, illam sectari. Is enim cum ex scientia suae artis coepisset, inflatus superbia, stultior apparere, quadam nocte assumpsere daemones poetarum species Virgilii et Horatii atque Juvenalis, apparentesque illi, fallaces retulerunt grates quoniam suorum dicta voluminum charius amplectens exerceret, seque illorum posteritatis felicem esse praeconem; promiserunt ei insuper suae gloriae postmodum fore participem. Hisque daemonum fallaciis depravatus, coepit multa turgide docere fidei sacrae contraria, dictaque poetarum per omnia credenda esse asserebat. Ad ultimum vero haereticus est repertus, atque a pontifice ipsius urbis Petro damnatus.

Plures etiam per Italiam tempore hujus pestiferi dogmatis reperti, quique ipsi aut gladiis aut incendiis perierunt. Ex Sardinia quoque insula, quae his plurimum abundare solet, ipso tempore aliqui egressi, partem populi in Hispania corrumpentes, et ipsi a viris catholicis exterminati sunt. Quod praesagium Joannis prophetiae congruit; quia dixit Satanam solvendum, expletis mille annis, de quibus in tertio jam libello prolixias [sic, read prolixius] tractabimus.

The translation may come from Rodulfus Glaber, Opera, edd. John France, Neithard Bulst, and Paul Reynolds (Oxford: Clarendon Press, 1989), pp. 92-93, which I haven't seen. The Latin text and a different translation can also be found in Jan M. Ziolkowski and Michael C.J. Putnam, edd., The Virgilian Tradition: The First Fifteen Hundred Years (New Haven: Yale University Press, 2008), pp. 895-896.