So far, I have only proven that God is the simple, necessary, eternal, unchangeable first cause of one particular change in the world. But there is a first cause for every single kind of change in the world. That includes the coming into existence of everything. So there are two options: either there is one God or there and many gods. God is one and this means that he is the first cause of all change in the universe.
God is one because God is simple. If two different things are different from each other, then they have different attributes. If two objects are the same in absolutely every way, then they are the same thing. Now God is simple. That means that his essence is the same thing as his existence. These facts apply to all Gods. So if there were more than one God, then the two Gods would have to have different attributes. There are only two kinds of differences that God could have. One God could have a different essence than the other, or one God could have some additional attribute that is not a part of his essence. If both of these possibilities are false, then God is one.
God cannot have a different essence than another God because his essence is the same thing as his existence. Both God’s essence are the same as their existence. Since existence applies only to an essence, there cannot be a difference between one existence and another existence unless the essence is different. But God’s essences are the same as his existence. Therefore, God cannot have a different essence than another God.
God cannot have a different additional attribute than another God because God cannot have additional attributes at all. God cannot have additional attributes because he is an unchanging being. An attribute has two possibilities. First, it could be entailed by the essence of a being. That means that every being with that kind of essence has that attribute. Second, it could not be entailed by the essence. That means that this particular being might not have had that attribute. Let’s call attributes of the first kind properties and attributes of the second kind accidents. God cannot have properties because he is simple – remember that there is no part of what God is that is different from his essence. For the same reason, God cannot have accidents either. There is also another reason why God cannot have accidents. God cannot have accidents because there must be a cause for the accident to exist. The first cause of that change must be God – since he is the first cause. Now if God is the first cause of the accident, then he is either the first cause by his essence or not. If he is the first cause of that accident by his essence, then that accident is necessary. But no accident is necessary. If he is not the first cause of his accident by essence, then he could have failed to have the accident. Since God’s essence is his existence, this just amounts to the claim that God is not the cause of the accident. But God is the first cause. Therefore, God has no accidents.
Since God has no accidents and there is no difference between two Gods on the basis of essence, there is only one God. This means that God is the first cause of all change in the world. He is also the cause of all things existing right now, and the cause of them continuing to exist. He is the cause of all past things and all future things, because he is eternal. He is the cause of all things that could exist because he is necessary. At this point, there should no longer be a problem calling this being ‘God’.