Petrus Alfonsi's Dialogus (written ca. 1109) combines polemics against Judaism and Islam with an original apology of Christian faith. The author argues that Judaism is not rational, whereas Christianity and Islam are, and that Islam is basically immoral. He supports his case with aggadic stories from the Talmud, pieces of Arabic learning, and a description of Muslim religious practice, all of which was new to his readers in Western Europe. The work had an immediate success, as the 35 extant manuscripts written before the Talmud trials testify. This paper tries to discern how readers of the High Middle Ages approached and understood the Dialogus, taking into account paratextual elements on the manuscripts, the context of transmission, and purposeful changes made by the scribes in the text. One version and two redactions of the Dialogus (one of them previously unknown) are analysed in some detail. It can be shown that the concentration on religious polemic becomes more intense over time, but there is also a noticeable awareness of the historical and ethical aspects of the text and interest for the story of Petrus Alfonsi's conversion.