assassin99:
Does the devil really exist? If so, why? What is the theological purpose of having an utterly evil being?
An utterly evil being is an impossibility - the devil is still good in origin. A ruined angel - which is what the devil is - has no capacity to be or become utterly evil, because it can’t de-create its being; that would take infinite power, and no created being has infinite power: and being & good are different terms for the same thing. So insofar as anything has being, it is good; insofar as it is good, it has being.
Only God is infinite in power, so only God can create. This is because God is infinite Goodness & infinite Being. The devil is not infinitely good, so, can’t annihilate itself. ##
No way ! The devil is ruined so completely that it barely exists - it’s in a very advanced state of decomposition (so to say). It has less reality about it than an atom, because an atom, though nearly nothing, is at least good as the almost nothing that it is; it’s not evil to be an atom. To be what the devil is, is an evil.
It’s very difficult to talk about evil - to do so is to try to talk sensibly about something that is senseless.
That is what damnation does - it leads to disintegration of one’s being, because it is by being related to God in His Love that created beings “hold together”. Take away God’s Love, and we fall to pieces. And it has rejected God’s Love, totally. What this means, can be glimpsed, very dimly, by considering what the saints would be, if they were damned; the devil is, basically, a ruined saint. And in losing God - deliberately, freely, knowingly, willingly - it has lost everything; for all that it could lose without ceasing to exist, it has.
So its condition is utterly anti-natural - it can’t do without God, but it is too ruined to be able to know God in the way it was created to do so. So it is utterly miserable. It’s like a dog with rabies: mad, bad, dangerous.
Does he have power in this world? Does the devil make me commit sins? If so, does that get me off the hook of responsibility for my actions? Is it a sin to not believe in the Devil?
1. One believes that there is a devil - one does not “believe in” the devil. There’s
an important difference, though I forget just what it is.
- The existence of the devil does not of itself relieve us of our accountability to God; temptation does not take away the ability to exercise free will. Sin is a free act, uncompelled by anything - which is why it is so serious: it is something that we are not forced to do; not even by the devil. So it won’t do to say “The devil made me do X” - no: I do X, it is the individual’s sin, not the devil’s.
Free Will is an almost God-like gift - it gives us the freedom to choose to respond to God’s Love, instead of being passively moved about by it: we are not ruled by necessity, but can influence the environment we live in, and be aware that we do so; including our spiritual environment. And where there is the freedom to respond to Love, there is the freedom to reject it. So it is a wonderful & terrifying gift.
- the devil is like a mad dog on a long leash - it’s chained, though it makes a hideous noise. We won’t be chewed by it, as long as we don’t put ourselves in a position to be chewed.
IMO, we flatter the devil by thinking much about it. Positive Love of God is better, not only because love for God will give us an allergy to wounding Him by sin (which is what “offending” Him is - “wounding” Him); but also because love, by its nature, strengthens love.
Besides, the victory of Christ on the Cross has shattered the power of the devil and abolished death - the Church’s work is to finish the mopping-up operations. So the thing called the devil is, for all purposes worth discussing, finished.
BTW - “majestic” is one word that does
not suit the devil
at all 
“Ridiculous”, possibly

The weakest lover of Christ is more majestic than the devil. It’s a bad joke, and a crashing bore

- as Cheserton says, it “fell by force of gravity”.