I.) It is possible for B to come from A and yet constitute A’s own being.
If John is a chef, he can be viewed as both the source of his recipes and whatever he cooks. But what he cooks can be taken from him and eaten while still leaving him as a chef, but his ideas of recipes cannot be taken from him and still leave him as a chef. It’s precisely in virtue of having such knowledge that he is a chef in the first place.
II.) Something whose whole existence is relative is possible.
The absolute and relative are contraries, so the relative is neither substance nor accident. Substance exists separately, accident exists dependently, relations co-exist.
1.) The Son comes forth from the Father, but he is not a creature. If it were necessary that all that came forth from God be a creature, then (I) is false.
2.) To remove the Son, even in thought, leaves absolutely nothing of the Father and vice-versa. Father-Son is the greatest possible revealing of what God is in himself, and it is possible that there be something whose whole existence is relative (II).
3.) The Father and Son are consubstantial. If the removal of something in an ontological category leaves nothing at all, not even in thought, then what was removed was not accidental.
4.) Father and Son are not modes of one person. Modes are accidents and so cannot be consubstantial.
5.) Father and Son are not many gods. What is consubstantial is not many in substance, and if the Father and Son were many gods then it would be possible, at least in thought, to have one exist without the other. For the same reason, Father and Son are not parts of some larger whole. All said here is against (II) via (2).