2 Peter 2,4

For if God did not spare the angels when they sinned, but condemned them to the chains of Tartarus and handed them over to be kept for judgment."

Teaching

According to Father Jean-Michel Garrigues, O.P.:

The Angels were created good by God, not only in nature but also in grace. They are much more perfect than us, and they know all perfections naturally known to God. However, that doesn't mean that they don't need faith. They know nothing about the Mystery of the Being of God, and His trinitary nature, because of the infinite distance between the created and the Increated, which the creature, including the angel, cannot cross. Thus, they are required to cultivate faith, exactly like us, and their faith must be freely willed and loving, exactly like us. They have "to consent to love." However, this consent was rejected by a few because of pride, with full knowledge of the consequences. This refusal turned those angels into devils, who prefer the sufferings of hell to the act of loving humility which, by freeing them from their ego, would have shattered the latter. They used the powers of their nature against God's plan. One would wonder why God didn't withdraw the powers He gave them; the answer is simply because "the gifts and the call of God are irrevocable." (Romans 11, 29)

In God's plan, "the older (the angel) will serve the younger (the man)" (Genesis 25,23), "so that no human being might boast before God." (1 Corinthians 1,29). As a consequence, He gave to the Angels enough grace to amount to their respective degrees of natural perfection, but He bestowed on men graces much greater than the measure of their natural capacities, as noted by St. Thomas of Aquinas. Therefore, man has the ability to become equal or even surpass the Angels in charity, thus in glory. After all, Saint Paul wrote that "[we] will judge Angels." (1 Corinthians 6,4) and Mary is honored with the title "Queen of the angels." This divine choice triggered Lucifer jealousy and his refusal to accept it  was the stumbling block which unleashed his demonic pride: "But by the envy of the devil, death entered the world, and they who are allied with him experience it." (Wisdom 2:24). Lucifer - whose name meant "bearer of light" - became Satan, "a murderer from the beginning" (John 8,44).

During our baptism vows, the Church asks is if we "reject Satan, all his works and all his empty promises." The Gospel reminds us that it's an angel who is "the father of lies" (John 8,44), "who has sinned from the beginning" (1 John 3,8). Man sinned from the beginning as well, but he did it "deceived by the serpent" (2 Corinthians 11,3), who is the author of sin. The devil cannot penetrate the human soul, as only God is "the searcher of hearts and minds" (Revelation 2,23), but he has an angelic power over the body (cf. 2nd book of Job). Through the imagination and wounded memory of the body, he lures it and tempts it by soliciting its free action, without being able to act directly on its will, as grace does. God allows this trial to strengthen our bond to Him. 

Prayer

Saint Michael Archangel,
defend us in battle,
be our protection against the wickedness and snares of the devil;
may God rebuke him, we humbly pray;
and do thou, O Prince of the heavenly host,
by the power of God, cast into hell
Satan and all the evil spirits
who prowl through the world seeking the ruin of souls.
Amen.

Prayer to St. Michael, by Pope Leo XIII