Perfidy toward Muslims was already a shared polemical trope
Evidence
- Reprobatio vi; Ricoldo, Disp. x11; SSM 1.iv.6. 58. Vitry (1.v) said that Muhammad taught that Muslims need not keep faith with their enemies, and San Pedro warned that Muslims would not observe oaths nisi ibi propriam utilitatem videant; ‘an accusation common among enemies’: e.g. used against each other by Catholics and Protestants. Most later travellers had something to say on the trustworthiness of Muslims (see bibliography). — Norman Daniel, Islam and the West, 2209