The philosophical problem how the essence of God can be defined and what this substance, if it can be called a substance at all, might be like is present in several treatises of the Corpus Hermeticum and in some philosophical and theological texts of late antiquity. In my essay I try to find the correct interpretation of the idiom οὐσία θεοῦ in the Hermetic texts with the help of some parallel writings from Jamblich and Sallustius. After the explanation of the relevant texts I conclude that the term οὐσία is used only to the cosmic gods, not to the first principle, and it has the function to connect the absolute transcendent first cause to the material world through the cosmic gods. The main source of this conception is the platonic tradition, as it can be seen not only from the similarity of the content, but from the similar use of the philosophical terms.