Objection 1. It seems that those who see the essence of God do not see all they see in Him at one and the same time. For according to the Philosopher (Topic. ii): "It may happen that many things are known, but only one is understood." But what is seen in God, is understood; for God is seen by the intellect. Therefore those who see God do not see all in Him at the same time.
Objection 2. Further, Augustine says (Gen. ad lit. viii, 22,23), "God moves the spiritual creature according to time"--i.e. by intelligence and affection. But the spiritual creature is the angel who sees God. Therefore those who see God understand and are affected successively; for time means succession.
On the contrary, Augustine says (De Trin. xvi): "Our thoughts will not be unstable, going to and fro from one thing to another; but we shall see all we know at one glance."
I answer that, What is seen in the Word is seen not successively, but at the same time. In proof whereof, we ourselves cannot know many things all at once, forasmuch as understand many things by means of many ideas. But our intellect cannot be actually informed by many diverse ideas at the same time, so as to understand by them; as one body cannot bear different shapes simultaneously. Hence, when many things can be understood by one idea, they are understood at the same time; as the parts of a whole are understood successively, and not all at the same time, if each one is understood by its own idea; whereas if all are understood under the one idea of the whole, they are understood simultaneously. Now it was shown above that things seen in God, are not seen singly by their own similitude; but all are seen by the one essence of God. Hence they are seen simultaneously, and not successively.
Reply to Objection 1. We understand one thing only when we understand by one idea; but many things understood by one idea are understood simultaneously, as in the idea of a man we understand "animal" and "rational"; and in the idea of a house we understand the wall and the roof.
Reply to Objection 2. As regards their natural knowledge, whereby they know things by diverse ideas given them, the angels do not know all things simultaneously, and thus they are moved in the act of understanding according to time; but as regards what they see in God, they see all at the same time.
Source: Summa Theologica (New Advent)