In the Ethics Spinoza attempts to give his philosophical metaphysics an absolute foundation in the style of Euclidean proofs. One of the principal focuses of the Ethics is to show that God is Nature and Nature is God. Like Descartes and Leibniz, Spinoza was a rationalist which meant he believed that all knowledge could be deduced from certain a priori self-evident truths.
The ancient Stoics would agree with Spinoza that God is Nature and Nature is God. The ancient Stoics went about many different means to prove the existence of God as a reasoning Universe. Zeno of Citium used something like the ontological argument to prove the Universe is a reasoning being. Zeno declared,
That which exercises reason is more excellent than that which does not exercise reason; there is nothing more excellent than the universe, therefore the universe exercises reason.De Natura Deorum by Cicero, ii. 8.; iii. 9.
Spinoza reasoned that everything is God because God is a substance with infinite mental and material attributes. Everything owes its existence to God because all our mental attributes and material attributes are all part of God. We are all tied together with the substance of Nature or God itself.
According to Spinoza, while we are absolutely determined both mentally and materially to be as we are, God or Nature is self-determined because God is a being from which all cause and effect relationships arise.
From God’s supreme power, or infinite nature, an infinite number of things – that is, all things have necessarily flowed forth in an infinite number of ways, or always flow from the same necessity; in the same way as from the nature of a triangle it follows from eternity and for eternity, that its three interior angles are equal to two right angles.Ethics, Part 1, XVII
God is truly an infinite being that necessitates all truths both material and mental events. All truths can be deduced from God the same way that it can be deduced from mathematics that three interior angles of a triangle are equal to two right angles.
It is a shame we do not have all the complete works of the ancient Stoics, but we know that they used a variety of methods to support their claim that the Universe is Divine and Providential. Zeno, for instance, used the ontological argument discussed earlier. Zeno also used the argument from design which is empirical. The Stoics were not afraid to use both a priori and a posteriori arguments that they had available.
One thing the Stoics never did use was the prime cause argument for the existence of God. This was probably because it was not popular among any of the Stoics to believe that God and Nature were separable. The Universe is the necessary being that necessitates all things throughout Itself.
As was demonstrated above, Spinoza and the Stoics were on the same page that God is Nature and Nature is God. I will leave you with a quote from Marcus Aurelius:
Constantly regard the universe as one living being, having one substance and one soul; and observe how all things have reference to one perception, the perception of this one living being.Meditations, IV, 40