The present article discusses the Muslim legal scholar and theologian `Al & amacr;' alDin al-B & amacr;ji (631-714/1233-1314) and his polemic against the Pentateuch, which he read in at least two Christian Arabic translations that were in use among R & umacr;m Orthodox Christians (Melkites). It aims to identify the recensions of the Pentateuch that al-B & amacr;ji had access to, and to understand how the differences between these recensions contributed to his view that the shared Jewish and Christian scripture had undergone changes. The article suggests that alB & aacute;ji used a combination of arguments to undermine especially the Christian reception of divine revelation, pointing out apparent inconsistencies and illogicalities in the biblical stories themselves as well as text -critical cruxes caused by discrepancies between different versions that circulated side by side within the Eastern Christian communities. Finally, some of the "irrationalities" he describes seem to be particular of the copies of the texts he had in front of him.