In accord with this ontology, the Stoics, like the Epicureans, make God material. But while the Epicureans think the gods are too busy being blessed and happy to be bothered with the governance of the universe (Epicurus, Letter to Menoeceus 123–4), the Stoic God is immanent throughout the whole of creation and directs its development down to the smallest detail. God is identical with one of the two ungenerated and indestructible first principles (archai) of the universe. One principle is matter which they regard as utterly unqualified and inert. It is that which is acted upon. God is identified with an eternal reason (logos, Diog. Laert. 44B ) or intelligent designing fire (Aetius, 46A) which structures matter in accordance with Its plan. This plan is enacted time and time again, beginning from a state in which all is fire, through the generation of the elements, to the creation of the world we are familiar with, and eventually back to fire in a cycle of endless recurrence. The designing fire of the conflagration is likened to a sperm which contains the principles or stories of all the things which will subsequently develop (Aristocles in Eusebius, 46G).
Under this guise, God is also called ‘fate.’ It is important to realise that the Stoic God does not craft its world in accordance with its plan from the outside, as the demiurge in Plato's Timaeus is described as doing. Rather, the history of the universe is determined by God's activity internal to it, shaping it with its differentiated characteristics. The biological conception of God as a kind of living heat or seed from which things grow seems to be fully intended. The further identification of God with pneuma or breath may have its origins in medical theories of the Hellenistic period. See Baltzly (2003).
Stoicism (Stanford Encyclopedia of Philosophy)