1.42 What’s this business about fallen angels?
The Bible tells us how evil was caused by the fall into sin of an angel, who became Satan or the devil. This angel chose to oppose God out of his own free will, and rejected God’s love. The devil can do a great deal of harm, but he remains a creature. Therefore, his power is not infinite. He cannot stop the coming of God’s kingdom.
Only God knows why he allows the evil work of the devil to continue, and why evil still exists in the world. However, we have been promised that in all things God works for the good of those who love him Rom. 8:28: And we know that in all things God works for the good of those who love him, who are called according to his purpose..
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The Wisdom of the Church
What was the fall of the angels?
This expression indicates that Satan and the other demons, about which Sacred Scripture and the Tradition of the Church speak, 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. They try to associate human beings with their revolt against God. However, God has wrought in Christ a sure victory over the Evil One. [CCCC 74]
This is what the Church Fathers say
The true cause of the blessedness of the good angels is found to be this, that they cleave to him who supremely is. And if we ask the cause of the misery of the bad, it occurs to us, and not unreasonably, that they are miserable because they have forsaken him who supremely is, and have turned to themselves who have no such essence. [St. Augustine, The City of God, bk 12, chap. 6 (ML 41, 353)]