The book later printed as De Mysteriis presents itself as the reply of an Egyptian priest, Abamon, to a sceptical letter. Both halves of the frame are real. The letter survives in quotation: Porphyry's Letter to Anebo, a barrage of pointed questions about why gods should need incense, why they descend at the summons of a ritualist, and why the pure should traffic with matter at all.
The answer is by Iamblichus, and the Egyptian mask was part of the argument, claiming for ritual the authority of a wisdom older than Greece. The reply concedes the premise and reverses the conclusion. Precisely because the human soul has descended fully into the body, Iamblichus argues, it cannot think its way back; help must come from the gods, and the gods have provided it in the form of rites, symbols and material tokens that carry their power. Theurgy on this account is not magicians compelling heaven but heaven condescending to work through appointed signs, and Iamblichus is careful to say the gods are not moved by the rites, but work through them. The book also systematises: kinds of divine beings, kinds of divination, degrees of vision, sorted with a schoolman's patience. Its afterlife was long. The emperor Julian took Iamblichus as the master voice of a restored paganism, and when Ficino translated the work in the Renaissance he gave it the title it still wears.