CCC 391 Behind the disobedient choice of our first parents lurks a seductive voice, opposed to God, which makes them fall into death out of envy.1 Scripture and the Church’s Tradition see in this being a fallen angel, called “Satan” or the “devil”.2 The Church teaches that Satan was at first a good angel, made by God: “The devil and the other demons were indeed created naturally good by God, but they became evil by their own doing.”3

1 Cf. Gen 3:1-5; Wis 2:24.
2 Cf Jn 8:44; Rev 12:9.
3 Lateran Council IV (1215): DS 800.