Objection 1. It seems that there are accidents in God. For substance cannot be an accident, as Aristotle says (Phys. i). Therefore that which is an accident in one, cannot, in another, be a substance. Thus it is proved that heat cannot be the substantial form of fire, because it is an accident in other things. But wisdom, virtue, and the like, which are accidents in us, are attributes of God. Therefore in God there are accidents.
Objection 2. Further, in every genus there is a first principle. But there are many "genera" of accidents. If, therefore, the primal members of these genera are not in God, there will be many primal beings other than God--which is absurd.
On the contrary, Every accident is in a subject. But God cannot be a subject, for "no simple form can be a subject", as Boethius says (De Trin.). Therefore in God there cannot be any accident.
I answer that, From all we have said, it is clear there can be no accident in God.
First, because a subject is compared to its accidents as potentiality to actuality; for a subject is in some sense made actual by its accidents. But there can be no potentiality in God, as was shown (2, 3).
Secondly, because God is His own existence; and as Boethius says (Hebdom.), although every essence may have something superadded to it, this cannot apply to absolute being: thus a heated substance can have something extraneous to heat added to it, as whiteness, nevertheless absolute heat can have nothing else than heat.
Thirdly, because what is essential is prior to what is accidental. Whence as God is absolute primal being, there can be in Him nothing accidental. Neither can He have any essential accidents (as the capability of laughing is an essential accident of man), because such accidents are caused by the constituent principles of the subject. Now there can be nothing caused in God, since He is the first cause. Hence it follows that there is no accident in God.
Reply to Objection 1. Virtue and wisdom are not predicated of God and of us univocally. Hence it does not follow that there are accidents in God as there are in us.
Reply to Objection 2. Since substance is prior to its accidents, the principles of accidents are reducible to the principles of the substance as to that which is prior; although God is not first as if contained in the genus of substance; yet He is first in respect to all being, outside of every genus.
Source: Summa Theologica (New Advent)