The position of the Ikhwān al-Ṣafāʼ on other religions has often been characterized as remarkably tolerant. The aim of this study is to investigate, firstly, how the Ikhwān al-Ṣafāʼ explain the phenomenon of religious diversity, and, secondly, in what sense and to what extent they recognize other religions as valid. The 42nd epistle may be interpreted as the Ikhwān’s answer to the problem of religious scepticism induced by the doctrinal diversity of their time. Their stance on religious diversity is genuinely pluralistic in that they regard all religions, at least to some extent, to be valid means for approaching God. This pluralistic outlook is founded on multiple grounds. Firstly, the distinction between exoteric and esoteric religion. Secondly, a minimalistic interpretation of what constitutes the core of religious belief. And thirdly, the assessment of doctrinal divergence as a necessary, and even beneficial, part of the progression towards the truth.