From the Catholic Encyclopedia under "Devil," is this interesting passage:


"Although nothing definite can be known as to the precise nature of the probation of the angels and the manner in which many of them fell, many theologians have conjectured, with some show of probability, that the mystery of the Divine Incarnation was revealed to them, that they saw that a nature lower than their own was to be hypostatically united to the Person of God the Son, and that all the hierarchy of heaven must bow in adoration before the majesty of the Incarnate Word; and this, it is supposed, was the occasion of the pride of Lucifer (cf. Suarez, De Angelis, lib. VII, xiii). As might be expected, the advocates of this view seek support in certain passages of Scripture, notably in the words of the Psalmist as they are cited in the Epistle to the Hebrews: "And again, when he bringeth in the first-begotten into the world, he saith: And let all the angels of God adore Him" (Hebrews 1:6; Psalm 96:7). And if the twelfth chapter of the Apocalypse may be taken to refer, at least in a secondary sense, to the original fall of the angels, it may seem somewhat significant that it opens with the vision of the Woman and her Child. But this interpretation is by no means certain, for the text in Hebrews 1, may be referred to the second coming of Christ, and much the same may be said of the passage in the Apocalypse.

It would seem that this account of the trial of the angels is more in accordance with what is known as the Scotist doctrine on the motives of the Incarnation than with the Thomist view, that the Incarnation was occasioned by the sin of our first parents. For since the sin itself was committed at the instigation of Satan, it presupposes the fall of the angels. How, then, could Satan's probation consist in the fore-knowledge of that which would, ex hypothesi, only come to pass in the event of his fall? In the same way it would seem that the aforesaid theory is incompatible with another opinion held by some old theologians, to wit, that men were created to fill up the gaps in the ranks of the angels. For this again supposes that if no angels had sinned no men would have been made, and in consequence there would have been no union of the Divine Person with a nature lower than the angels.

As might be expected from the attention they had bestowed on the question of the intellectual powers of the angels, the medieval theologians had much to say on the time of their probation. The angelic mind was conceived of as acting instantaneously, not, like the mind of man, passing by discursive reasoning from premises to conclusions. It was pure intelligence as distinguished from reason. Hence it would seem that there was no need of any extended trial. And in fact we find St. Thomas and Scotus discussing the question whether the whole course might not have been accomplished in the first instant in which the angels were created. The Angelic Doctor argues that the Fall could not have taken place in the first instant. And it certainly seems that if the creature came into being in the very act of sinning the sin itself might be said to come from the Creator. But this argument, together with many others, is answered with his accustomed acuteness by Scotus, who maintains the abstract possibility of sin in the first instant. But whether possible or not, it is agreed that this is not what actually happened. For the authority of the passages in Isaiah and Ezekiel, which were generally accepted as referring to the fall of Lucifer, might well suffice to show that for at least one instant he had existed in a state of innocence and brightness. To modern readers the notion that the sin was committed in the second instant of creation may seem scarcely less incredible than the possibility of a fall in the very first. But this may be partly due to the fact that we are really thinking of human modes of knowledge, and fail to take into account the Scholastic conception of angelic cognition. For a being who was capable of seeing many things at once, a single instant might be equivalent to the longer period needed by slowly-moving mortal."

http://www.newadvent.org/cathen/04764a.htm