4 The devil then is the first author of sin, and the father of the wicked: and this is the Lord's saying, not mine, that the devil sins from the beginning: none sinned before him. But he sinned, not as having received necessarily from nature the propensity to sin, since then the cause of sin is traced back again to Him that made him so; but having been created good, he has of his own free will become a devil, and received that name from his action.
For being an Archangel he was afterwards called a devil from his slandering: from being a good servant of God he has become rightly named Satan; for “Satan” is interpreted the adversary. And this is not my teaching, but that of the inspired prophet Ezekiel: for he takes up a lamentation over him and says, You were a seal of likeness, and a crown of beauty; in the Paradise of God were you born: and soon after, You were born blameless in your days, from the day in which you were created, until your iniquities were found in you. Very rightly has he said, were found in you; for they were not brought in from without, but you yourself begot the evil.
The cause also he mentions immediately: Your heart was lifted up because of your beauty: for the multitude of your sins were you wounded, and I did cast you to the ground. In agreement with this the Lord says again in the Gospels: I beheld Satan as lightning fall from heaven. You see the harmony of the Old Testament with the New. He when cast out drew many away with him. It is he that puts lusts into them that listen to him: from him come adultery, fornication, and every kind of evil. Through him our forefather Adam was cast out for disobedience, and exchanged a Paradise bringing forth wondrous fruits of its own accord for the ground which brings forth thorns.
Source: Catechetical Lectures (New Advent)