Jesus rebukes Peter at Caesarea Philippi by calling him ‘Satan’. The redactor of Matthew adds to this: ‘You are a stumbling block for me!’ This article argues that this editorial addition reflects a tendency in Jewish sources (1) to use the image of ‘stumbling’ to talk of sin, and (2) to identify the diabolic figure (e.g. Satan, Belial, Evil Inclination) as the cause of ‘stumbling’, earning it the title ‘stumbling block’. This tendency has its origins in the Hebrew Bible, is clearly expressed in the literature of the Qumran community, and is further developed in rabbinic sources.