III. By SUBSTANCE (substantia) I understand that which is in itself and is conceived through itself: that is, that, the conception of which does not depend on the conception of another thing, from which conception it must be formed.From there, he not only proves that God must exist, he proves that only God must exist, and what is more, only God does exist. So, God is everything. Therefore, God doesn't have intellect or will. And so on.
I found on the web some folks that postulated that Spinoza was the first panentheist. That is not right because penentheism believes that God is everything and more. Also, while panentheism provides some challenges for the notion that God acts in the lives of people in response to prayer (at least it does for me), it suggests a conscious knowing universal God. What that means for beings that are a part of that knowing loving God, is food for thought.
I had heard people suggest that Spinoza's proof is actually tongue in cheek--that it is intended to prove too much and therefore really be a proof against the existence of God. I think that is not an unreasonable reading. I'm anxious to move on to his books regarding mortal ethics. This book seems to delightfully prove God out of relevance if not out of existence. As a student of rhetoric and persuasion, I can't help but admire that.