This paper argues that Spinoza makes a distinction between the constitutive essence of God (the totality of His attributes) and the essence of God per se (His power and causal efficacy). Using this distinction, I explain how Spinoza can conceive of God as being both an immutable simple unity and a subject for constantly changing modes. Spinoza believes that God qua Natura Naturans is immutable while God qua Natura Naturata is not. With this point established, Curley's claim that Spinozistic modes are causally dependent on but not properties of God loses much of its attraction. In conclusion, I suggest how God's essence is related to His attributes and His modes.