Each particular attribute of the one substance must be conceived through itself. God, or substance, consisting of infinite attributes, of which each expresses eternal and infinite essentiality, necessary exists. Therefore God necessarily exists. NOTE that Spinoza gives more proofs for the existence of God here at Prop 11. No attribute of substance can be conceived from which it would follow that substance can be divided.
Substance absolutely infinite is indivisible.
Besides God no substance can be granted or conceived. Whatever is, is in God, and without God, nothing can be or be conceived. God acts solely by the laws of his own nature, and is not constrained by anyone. God is the indwelling and not the transient cause of all things. God, and all the attributes of God, are eternal.