Objection 1. It would seem that the first man's pride did not consist in his coveting the Divine likeness. For no one sins by coveting that which is competent to him according to his nature. Now God's likeness is competent to man according to his nature: for it is written (Genesis 1:26): "Let us make man to our image and likeness." Therefore he did not sin by coveting God's likeness.
Objection 2. Further, it would seem that man coveted God's likeness in order that he might obtain knowledge of good and evil: for this was the serpent's suggestion: "You shall be as Gods knowing good and evil." Now the desire of knowledge is natural to man, according to the saying of the Philosopher at the beginning of his Metaphysics i, 1: "All men naturally desire knowledge." Therefore he did not sin by coveting God's likeness.
Objection 3. Further, no wise man chooses the impossible. Now the first man was endowed with wisdom, according to Sirach 17:5, "He filled them with the knowledge of understanding." Since then every sin consists in a deliberate act of the appetite, namely choice, it would seem that the first man did not sin by coveting something impossible. But it is impossible for man to be like God, according to the saying of Exodus 15:11, "Who is like to Thee among the strong, O Lord?" Therefore the first man did not sin by coveting God's likeness.
On the contrary, Augustine commenting on Psalm 68:5 [Enarr. in Ps. 68, "Then did I restore [Douay: 'pay'] that which I took not away," says: "Adam and Eve wished to rob the Godhead and they lost happiness."
I answer that, likeness is twofold. One is a likeness of absolute equality [Cf. I, 93, 1: and such a likeness to God our first parents did not covet, since such a likeness to God is not conceivable to the mind, especially of a wise man.
The other is a likeness of imitation, such as is possible for a creature in reference to God, in so far as the creature participates somewhat of God's likeness according to its measure. For Dionysius says (Div. Nom. ix): "The same things are like and unlike to God; like, according as they imitate Him, as far as He can be imitated; unlike, according as an effect falls short of its cause." Now every good existing in a creature is a participated likeness of the first good.
Wherefore from the very fact that man coveted a spiritual good above his measure, as stated in the foregoing Article, it follows that he coveted God's likeness inordinately.
It must, however, be observed that the proper object of the appetite is a thing not possessed. Now spiritual good, in so far as the rational creature participates in the Divine likeness, may be considered in reference to three things. First, as to natural being: and this likeness was imprinted from the very outset of their creation, both on man--of whom it is written (Genesis 1:26) that God made man "to His image and likeness"--and on the angel, of whom it is written (Ezekiel 28:12): "Thou wast the seal of resemblance." Secondly, as to knowledge: and this likeness was bestowed on the angel at his creation, wherefore immediately after the words just quoted, "Thou wast the seal of resemblance," we read: "Full of wisdom." But the first man, at his creation, had not yet received this likeness actually but only in potentiality. Thirdly, as to the power of operation: and neither angel nor man received this likeness actually at the very outset of his creation, because to each there remained something to be done whereby to obtain happiness.
Accordingly, while both (namely the devil and the first man) coveted God's likeness inordinately, neither of them sinned by coveting a likeness of nature. But the first man sinned chiefly by coveting God's likeness as regards "knowledge of good and evil," according to the serpent's instigation, namely that by his own natural power he might decide what was good, and what was evil for him to do; or again that he should of himself foreknow what good and what evil would befall him. Secondarily he sinned by coveting God's likeness as regards his own power of operation, namely that by his own natural power he might act so as to obtain happiness. Hence Augustine says (Gen. ad lit. xi, 30) that "the woman's mind was filled with love of her own power." On the other hand, the devil sinned by coveting God's likeness, as regards power. Wherefore Augustine says (De Vera Relig. 13) that "he wished to enjoy his own power rather than God's." Nevertheless both coveted somewhat to be equal to God, in so far as each wished to rely on himself in contempt of the order of the Divine rule.
Reply to Objection 1. This argument considers the likeness of nature: and man did not sin by coveting this, as stated.
Reply to Objection 2. It is not a sin to covet God's likeness as to knowledge, absolutely; but to covet this likeness inordinately, that is, above one's measure, this is a sin. Hence Augustine commenting on Psalm 70:18, "O God, who is like Thee?" says: "He who desires to be of himself, even as God is of no one, wishes wickedly to be like God. Thus did the devil, who was unwilling to be subject to Him, and man who refused to be, as a servant, bound by His command."
Reply to Objection 3. This argument considers the likeness of equality.
Source: Summa Theologica (New Advent)