The paper will examine the 'secret knowledge' as is presented in the al-Risala al-Jāmiʿa, the so-called 'crown' (i.e., an esoteric explanation) of the epistles of the Brethren of Purity (al-Ikhwān al-Ṣafāʾ). While the Ismaili commitment of the Brethren is not unanimously accepted, scholars generally agree in recognising the Jāmiʿa as Ismaili. The doctrinal content of this work is reserved for initiates of a higher level with respect to those approaching the encyclopaedia. It mainly refers to philosophical-religious doctrines, but also to religious practices: the relationship with Epistle 50 of the Ikhwān al-Ṣafāʾ is significant with this regard. Whether or not they are the authors of the encyclopedia themselves, or related to them, the author(s) of the Jāmiʿa — just like the Brethren of Purity — appear to be an 'ideological minority', characterised by a philosophical approach of extra-Islamic origin, and an esoteric interpretation of the Holy Book that marks their diversity with respect to the so-called orthodoxy. It will be examined how ancient doctrines of Greek origin are elaborated in the light of the Islamic religion and interpreted as stemming from an esoteric interpretation of the Holy Book. This will provide an additional opportunity for comparisons with the reworking and use of ancient doctrines in the encyclopedia of the Ikhwān al-Ṣafāʾ.