Objection 1. It would seem that the angels know future events. For angels are mightier in knowledge than men. But some men know many future events. Therefore much more do the angels.
Objection 2. Further, the present and the future are differences of time. But the angel's intellect is above time; because, as is said in De Causis, "an intelligence keeps pace with eternity," that is, aeviternity. Therefore, to the angel's mind, past and future are not different, but he knows each indifferently.
Objection 3. Further, the angel does not understand by species derived from things, but by innate universal species. But universal species refer equally to present, past, and future. Therefore it appears that the angels know indifferently things past, present, and future.
Objection 4. Further, as a thing is spoken of as distant by reason of time, so is it by reason of place. But angels know things which are distant according to place. Therefore they likewise know things distant according to future time.
On the contrary, Whatever is the exclusive sign of the Divinity, does not belong to the angels. But to know future events is the exclusive sign of the Divinity, according to Isaiah 41:23: "Show the things that are to come hereafter, and we shall know that ye are gods." Therefore the angels do not know future events.
I answer that, The future can be known in two ways.
First, it can be known in its cause. And thus, future events which proceed necessarily from their causes, are known with sure knowledge; as that the sun will rise tomorrow. But events which proceed from their causes in the majority of cases, are not known for certain, but conjecturally; thus the doctor knows beforehand the health of the patient. This manner of knowing future events exists in the angels, and by so much the more than it does in us, as they understand the causes of things both more universally and more perfectly; thus doctors who penetrate more deeply into the causes of an ailment can pronounce a surer verdict on the future issue thereof. But events which proceed from their causes in the minority of cases are quite unknown; such as casual and chance events.
In another way future events are known in themselves. To know the future in this way belongs to God alone; and not merely to know those events which happen of necessity, or in the majority of cases, but even casual and chance events; for God sees all things in His eternity, which, being simple, is present to all time, and embraces all time. And therefore God's one glance is cast over all things which happen in all time as present before Him; and He beholds all things as they are in themselves, as was said before when dealing with God's knowledge (14, 13). But the mind of an angel, and every created intellect, fall far short of God's eternity; hence the future as it is in itself cannot be known by any created intellect.
Reply to Objection 1. Men cannot know future things except in their causes, or by God's revelation. The angels know the future in the same way, but much more distinctly.
Reply to Objection 2. Although the angel's intellect is above that time according to which corporeal movements are reckoned, yet there is a time in his mind according to the succession of intelligible concepts; of which Augustine says (Gen. ad lit. viii) that "God moves the spiritual creature according to time." And thus, since there is succession in the angel's intellect, not all things that happen through all time, are present to the angelic mind.
Reply to Objection 3. Although the species in the intellect of an angel, in so far as they are species, refer equally to things present, past, and future; nevertheless the present, past, and future do not bear the same relations to the species. Present things have a nature according to which they resemble the species in the mind of an angel: and so they can be known thereby. Things which are yet to come have not yet a nature whereby they are likened to such species; consequently, they cannot be known by those species.
Reply to Objection 4. Things distant according to place are already existing in nature; and share in some species, whose image is in the angel; whereas this is not true of future things, as has been stated. Consequently there is no comparison.
Source: Summa Theologica (New Advent)