Individual things are modifications of God's attributes
Formal Statement
Individual things are nothing but modifications of the attributes of God, or modes by which the attributes of God are expressed in a fixed and definite manner. God is the efficient cause of both the essence and the existence of things (P25), and particular things are how God's attributes are determinately configured.
In Plain Language
This is where the metaphysics lands on the ground. Every rock, every person, every thought — each is a "fixed and definite" way that one of God's attributes is expressed. You are not a substance in your own right; you are a pattern in the fabric of divine nature. This is not a demotion — it is a relocation. Finite things are real, but their reality is the reality of modes, not of independent substances. The corollary to P25 is the hinge that lets Spinoza pivot from pure metaphysics toward the study of particular beings: minds, bodies, affects.
Why This Follows
P15 (gs-14) established that everything is in God. P16 (gs-15) established that infinitely many things follow from God's nature necessarily. P25 specifies that God is the cause of both the essence and existence of things. The corollary draws the conclusion: individual things just are determinate modifications of God's attributes, connecting the abstract metaphysics to the concrete world of particular beings.
Every individual thing is a determinate mode of one of God's attributes.
Connected Concepts
If you are a modification of God's attributes, what does that imply about your individuality — is it real or merely apparent?