Having seen the film adaptation of Dan Brown's novel Angels and Demons, I'd say many folks find themselves in his camp: believing that good and evil reside in the hearts of people, which is plenty to deal with. It's tough to distinguish between these spirits, because evil would hardly be successful unless it were disguised as good. Dragging a supernatural element into the complex moral one might only muddy the waters of human culpability. So what does the church say about beings celestial and infernal?

Angels come first in the discussion, because they literally came first. God made only one kind of purely spiritual being. Unlike mortals, created at a belated stage of the story, angels are incorporeal, immortal creatures. But like us they received the gift of free will and could choose once: to serve or not serve God.

Traditions from outside scripture (but hinted at in many biblical passages) tell us how the decision unfolded. Most angels chose to serve. One, however, refused and drew others to join the revolt against God: by definition, the revolt against goodness. Hell came into existence as the end result of choosing to absent oneself from God's presence and good purpose.

So Satan and his demons are in fact angels, though we know them better as "devils." Their existence is geared to associate human beings with revolt against God. Meanwhile the heavenly host is dedicated to God's service in a variety of ways. Some serve celestially in the divine liturgy, offering endless praise and glory to God. Others serve as messengers, guardians, and protectors for the sake of humanity. In Judaism, Christianity, and Islam the teachings and traditions about angels are fairly consistent and complementary.

When Jesus proclaims the coming of the kingdom of God, he accompanies and ratifies this proclamation by casting out demons. Demons know who Jesus is, and they flee from his presence. That reminds us we have no reason to fear the demonic if we associate ourselves with God's presence and purpose revealed in Jesus.

Two things to keep in mind regarding angels and demons: 1. We are freer than they are because we can freely choose to serve or not serve God continually throughout our lives. 2. Because we are free, we can never blame "the devil" for anything we do. What we make of each decision is up to us.

Scripture
Book of Tobit; Job 1:1-2:10; Isaiah 6:1-8; Daniel 3; Matthew 1:18-2:23; Luke 1:5-38; Hebrews 1:1-2:18; Book of Revelation

Online resource
"Angels" by Father Paul Turner

Books
A Catalogue of Angels: The Heavenly, the Fallen, and the Holy Ones Among Us by Vinita Hampton Wright (Paraclete Press, 2006)
The Angels and Their Mission: According to the Fathers of the Church by Jean Danielou, S.J. (Sophia Institute Press, 2009)