The subject is taken from the Golden Legend (Legenda Aurea, II, col. 50) where in the life of Saint Bernard is told how the aaint converted an excommunicated duke of Aquitaine by confronting him with the host on a paten outside a Church. The duke felt his limbs tremble, prostrated himself and thenceforth did as he was told by the Saint. Saint Bernard (c. 1090-1153) was a Cistercian monk and Abbot of Clairvaux; the Duke concerned came to be identified as Saint William of Aquitaine, but he died c. 812.
As Vlieghe shows, the composition was clearly inspired by Rubens Conversion of Saint Bavo, for St. Bavo's, Ghent, for which Rubens received payment in 1624 and by the unfinished Miracles of Saint Benedict in Brussels (see H. Vlieghe, Corpus Rubenianum Ludwig Burchard, Part VIII, Saints I, London and New York, 1972, nos. 72 & 73). De Crayer may have been given the latter painting by Rubenss widow as a gesture of thanks for his intervention in the disposal of paintings from Rubens estate. A preparatory sketch is in the Musée Bonnat, Bayonne (Vlieghe, no. A222), which differs chiefly in the disposition of the foreground groups.
The present work may be that recorded in the church of the Cistercian abbey of Villers-la-Ville by G.P. Mensaert, le Peintre Amateur et Curieux, Brussels, II, 1763, p. 88: 'Deux beaux tableaux peints par G. de Crayer font l'ornement de cette abbaye; ils représentent ... La conversion de Saint Guillaume ...'. Alternatively the latter could be the replica of the present work in the Catholic Cathedral of Louisville, Kentucky (Vlieghe, no. A224), which left Belgium in 1815 or 1820.