Whatever is, is in God
Formal Statement
Whatever is, is in God, and without God nothing can be or be conceived. Since God is the only substance (P14) and everything is either substance or mode (Ax.1), every existing thing is either God or a mode of God. Modes exist in substance and are conceived through it (Def.5). Therefore everything that exists, exists in God.
In Plain Language
This is Spinoza's immanence thesis, and it is breathtaking in scope. You, this book, every particle, every idea — all of it is "in God." Not in the sense of being inside a container, but in the sense of being a modification of the one substance. Nothing is outside God, because there is nothing outside substance, because there is only one substance. This is the proposition that makes Spinoza's philosophy a form of radical immanence rather than a dualism of God and world.
Why This Follows
P14 (gs-13) established that God is the only substance. Axiom 1 (gs-05) states that everything exists either in itself or in another. Everything that is substance is God; everything else is a mode, which by Def.5 (gs-03) exists in substance. So all things are in God — there is nowhere else to be.
All beings are either God (substance) or modes of God — nothing exists outside God.
Connected Concepts
If everything is "in God," does the distinction between sacred and secular, or between natural and supernatural, still make any sense?