According to that work, he stated that his religious doubts arose in 1295 when he treated a number of Jews for distress following their involvement in the failed messianic movement in Avila.
As Abner tells it, he "had a dream" in which a similar experience of crosses mysteriously appearing on his garments drove him to question his ancestral faith.
[1] Not being of those contented ones who, as Moses Narboni observes in his Maamar ha-Beḥirah (Essay on the Freedom of the Will; quoted by Grätz, p. 488), are satisfied with a peck of locust beans from one Friday to another, he resolved to embrace Christianity.
His most important work, the Moreh Zedek (Teacher of Righteousness), which now survives only in a 14th-century Castilian translation as Mostrador de Justicia, is one of the longest and most elaborate polemics against Judaism ever written and is considered one of the key sources for the history of anti-Jewish thought in fourteenth century Western Europe.
Abner/Alfonso's text rivals (and in many ways surpasses) the Ramon Martí's Pugio Fidei in length, complexity, variety of sources and psychological impact, although there is no evidence that Abner/Alfonso actually knew of the polemical Dominican's work.
Both his conversion and this defence of determinism aroused protests from his Jewish former study-partner, Isaac Pulgar, marked by great bitterness.