Typically, we think of two modes of predication regarding being: X might be predicated of Y as to substance, or as to accident. Augustine revises this formulation:
“Mais en Dieu il n’y a rien d’accidentel, parce qu’il est souverainement immuable, et néanmoins tout ce qui s’énonce de lui, ne s’énonce point de la substance. Ainsi nous distinguons en Dieu le Père d’avec le Fils, et le Fils d’avec le Père; et toutefois nous ne, disons pas qu’en eux cette distinction soit accidentelle, parce qu’éternellement l’un est Père, et l’autre est Fils.”
—De Trin. V.5.6
Creatures have no substance, properly speaking: what they have are intrinsic or incidental accidents (it’s appropriate to call the former “accidents,” because even these are subject to change in quality). God has no accidents, but it doesn’t therefore follow that everything predicated of him applies to his substance, because we affirm the consubstantiality of Father and Son, but distinguish them by relations. This brings up a fascinating point: relationships are accidents for creatures; for God, relationships are the sole mode in which his essence is realized.