Rather than a sin, I think of it more as a weak faith.
I will use myself as an example.
By many standards, I have ample reason to walk around depressed all day, every day. I've posted about most of it, but not all. Some days I get overwhelmed by it all and a mild form of self pity starts creeping in. That's when I know I've started trying to manage without God. I fully believe that God is carrying me though this enough that I am able to joke and laugh and be content with whatever I have, but still push on because I know that God has more in store for me.
Some days, that last thought before drifting off to sleep is "well, at least I have God". As, lonely as that sounds, He is more than enough for me. I can't complain too much about my situation because I know there are people that would kill for the luxuries that I, as an American, take for granted.
The point is that when God is with me, no matter where I am or what state I'm in, I know I'm never alone. It reminds me of this verse:
Romans 8
[sup]35[/sup] Who shall separate us from the love of Christ? Shall trouble or hardship or persecution or famine or nakedness or danger or sword? [sup]36[/sup] As it is written:
“For your sake we face death all day long;
we are considered as sheep to be slaughtered.”[sup][a][/sup]
[sup]37[/sup] No, in all these things we are more than conquerors through him who loved us. [sup]38[/sup] For I am convinced that neither death nor life, neither angels nor demons,[sup][b][/sup] neither the present nor the future, nor any powers, [sup]39[/sup] neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.
I remember reading this a long time ago and didn't give it much thought, but now I live it everyday. It's true that nothing in life compares to God's love -- nothing. I can only speak for myself, but there was a day a couple years ago where I was awakened to who God is and my purpose in him. I knocked when no other doors were there, and he did answer. That's the hope that keeps me going day to day. He put that in me, because it honestly wasn't there before.
(end essay)
So to answer your question, depression, to me, is a believer still clinging on to that last thread of rope before they reach for God's hand. Time and submission to God's will, is the cure. :)