Objection 1. It would seem that the devil did not desire to be as God. For what does not fall under apprehension, does not fall under desire; because the good which is apprehended moves the appetite, whether sensible, rational, or intellectual; and sin consists only in such desire. But for any creature to be God's equal does not fall under apprehension, because it implies a contradiction; for if the finite equals the infinite, then it would itself be infinite. Therefore an angel could not desire to be as God.
Objection 2. Further, the natural end can always be desired without sin. But to be likened unto God is the end to which every creature naturally tends. If, therefore, the angel desired to be as God, not by equality, but by likeness, it would seem that he did not thereby sin.
Objection 3. Further, the angel was created with greater fulness of wisdom than man. But no man, save a fool, ever makes choice of being the equal of an angel, still less of God; because choice regards only things which are possible, regarding which one takes deliberation. Therefore much less did the angel sin by desiring to be as God.
On the contrary, It is said, in the person of the devil (Isaiah 14:13-14), "I will ascend into heaven... I will be like the Most High." And Augustine (De Qu. Vet. Test. cxiii) says that being "inflated with pride, he wished to be called God."
I answer that, Without doubt the angel sinned by seeking to be as God. But this can be understood in two ways: first, by equality; secondly, by likeness. He could not seek to be as God in the first way; because by natural knowledge he knew that this was impossible: and there was no habit preceding his first sinful act, nor any passion fettering his mind, so as to lead him to choose what was impossible by failing in some particular; as sometimes happens in ourselves. And even supposing it were possible, it would be against the natural desire; because there exists in everything the natural desire of preserving its own nature; which would not be preserved were it to be changed into another nature. Consequently, no creature of a lower order can ever covet the grade of a higher nature; just as an ass does not desire to be a horse: for were it to be so upraised, it would cease to be itself. But herein the imagination plays us false; for one is liable to think that, because a man seeks to occupy a higher grade as to accidentals, which can increase without the destruction of the subject, he can also seek a higher grade of nature, to which he could not attain without ceasing to exist. Now it is quite evident that God surpasses the angels, not merely in accidentals, but also in degree of nature; and one angel, another. Consequently it is impossible for one angel of lower degree to desire equality with a higher; and still more to covet equality with God.
To desire to be as God according to likeness can happen in two ways. In one way, as to that likeness whereby everything is made to be likened unto God. And so, if anyone desire in this way to be Godlike, he commits no sin; provided that he desires such likeness in proper order, that is to say, that he may obtain it of God. But he would sin were he to desire to be like unto God even in the right way, as of his own, and not of God's power. In another way one may desire to be like unto God in some respect which is not natural to one; as if one were to desire to create heaven and earth, which is proper to God; in which desire there would be sin. It was in this way that the devil desired to be as God. Not that he desired to resemble God by being subject to no one else absolutely; for so he would be desiring his own 'not-being'; since no creature can exist except by holding its existence under God. But he desired resemblance with God in this respect--by desiring, as his last end of beatitude, something which he could attain by the virtue of his own nature, turning his appetite away from supernatural beatitude, which is attained by God's grace. Or, if he desired as his last end that likeness of God which is bestowed by grace, he sought to have it by the power of his own nature; and not from Divine assistance according to God's ordering. This harmonizes with Anselm's opinion, who says [De casu diaboli, iv.] that "he sought that to which he would have come had he stood fast." These two views in a manner coincide; because according to both, he sought to have final beatitude of his own power, whereas this is proper to God alone.
Since, then, what exists of itself is the cause of what exists of another, it follows from this furthermore that he sought to have dominion over others; wherein he also perversely wished to be like unto God.
From this we have the answer to all the objections.
Source: Summa Theologica (New Advent)