We don't need to fear the devil but we should understand who he is. Here are the basics.
Catholics do not believe that there are two “superpowers” in the cosmos —God and the devil, like the light and dark side of the force. Rather, we believe there is only one power — Almighty God, who is all-good, and the creator of everything, including the devil.
So why is there a devil at all? The Compendium of the Catechism, promulgated in 2005, explains.
“Satan and the other demons,” it says, “were angels, created good by God. They were, however, transformed into evil because with a free and irrevocable choice they rejected God and his Kingdom, thus giving rise to the existence of hell” (No. 74).
God is love, and he created the universe out of love. All of the creatures show his love, but only two kinds of creatures can love him back: Angels and human beings. Angels are spiritual creatures, acknowledged by nearly every religion and every culture in history. Like us, they have reason and will — the ability to think and to choose. That means they can love.
It also means they can choose not to love.
We human beings live in time, and if we choose not to love God, our choice plays out in slow motion, through the decisions and actions of a lifetime. Angels live outside of time, in eternity. If they chose not to love, that was it: They rejected God in the eternal “now” of heaven and entered the eternal darkness of hell.