Objection 1. It seems that God does not know evil things. For the Philosopher (De Anima iii) says that the intellect which is not in potentiality does not know privation. But "evil is the privation of good," as Augustine says (Confess. iii, 7). Therefore, as the intellect of God is never in potentiality, but is always in act, as is clear from the foregoing (2), it seems that God does not know evil things.
Objection 2. Further, all knowledge is either the cause of the thing known, or is caused by it. But the knowledge of God is not the cause of evil, nor is it caused by evil. Therefore God does not know evil things.
Objection 3. Further, everything known is known either by its likeness, or by its opposite. But whatever God knows, He knows through His essence, as is clear from the foregoing (5). Now the divine essence neither is the likeness of evil, nor is evil contrary to it; for to the divine essence there is no contrary, as Augustine says (De Civ. Dei xii). Therefore God does not know evil things.
Objection 4. Further, what is known through another and not through itself, is imperfectly known. But evil is not known by God; for the thing known must be in the knower. Therefore if evil is known through another, namely, through good, it would be known by Him imperfectly; which cannot be, for the knowledge of God is not imperfect. Therefore God does not know evil things.
On the contrary, It is written (Proverbs 15:11), "Hell and destruction are before God [Vulgate: 'the Lord']."
I answer that, Whoever knows a thing perfectly, must know all that can be accidental to it. Now there are some good things to which corruption by evil may be accidental. Hence God would not know good things perfectly, unless He also knew evil things. Now a thing is knowable in the degree in which it is; hence since this is the essence of evil that it is the privation of good, by the fact that God knows good things, He knows evil things also; as by light is known darkness. Hence Dionysius says (Div. Nom. vii): "God through Himself receives the vision of darkness, not otherwise seeing darkness except through light."
Reply to Objection 1. The saying of the Philosopher must be understood as meaning that the intellect which is not in potentiality, does not know privation by privation existing in it; and this agrees with what he said previously, that a point and every indivisible thing are known by privation of division. This is because simple and indivisible forms are in our intellect not actually, but only potentially; for were they actually in our intellect, they would not be known by privation. It is thus that simple things are known by separate substances. God therefore knows evil, not by privation existing in Himself, but by the opposite good.
Reply to Objection 2. The knowledge of God is not the cause of evil; but is the cause of the good whereby evil is known.
Reply to Objection 3. Although evil is not opposed to the divine essence, which is not corruptible by evil; it is opposed to the effects of God, which He knows by His essence; and knowing them, He knows the opposite evils.
Reply to Objection 4. To know a thing by something else only, belongs to imperfect knowledge, if that thing is of itself knowable; but evil is not of itself knowable, forasmuch as the very nature of evil means the privation of good; therefore evil can neither be defined nor known except by good.
Source: Summa Theologica (New Advent)