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.
(St. Thomas Aquinas' Summa Theologica- question 163, article 2)