So the Lord said to Cain, “Why are you angry? And why has your countenance fallen? If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.” Genesis 4:6-7
The Onset
When God asks, Cain, “Why has your countenance fallen,” it is the first allusion to depression in the Bible. What makes it such a significant reference is that in this short passage we are able to examine the cause, the consequence, and amazingly, God’s cure for the disorder.
Before depression sets in, the Bible discloses that Cain was a selfish, prideful, and faithless man. The account in Genesis bears witness to this. Cain and Abel present offerings to the Lord and God was not pleased with Cain’s for the aforementioned reasons. While all of Cain’s traits contributed to the objectionable circumstances, the primary issue was his faithlessness.
By faith Abel offered to God a more excellent sacrifice than Cain… Hebrews 11:4a
The thing Abel did, he did by faith, and since God declares in Romans 10:17 that, “faith comes by hearing, and hearing by the word of God, ” we know that both Cain and Abel had communication with God. Regrettably only Abel was obedient — Abel did well (in his faithful obedience) and Cain did not do well (in his faithlessness).
“Do well.”
In the context of the story, we know that ‘do well’ does not mean to execute good deeds, work hard and redeem yourself, or even pull yourself up by the boot straps. 
The reality of doing well is to employ the faith God has given, obey His word, and as a result you will be accepted. You may be interested to know that he word ‘accepted’ here in the Hebrew is ‘seh-ayth’ and it actually means to be elevated or lifted up. One might even say that you will be ‘un-depressed’ by the healing hand of God. Focus on making God happy rather than yourself, and watch what happens!
In steep contrast, God warns that ‘sin lies at the door’ for those who reject the cure, the consequences of which bring you lower, intensifying the despair. In other words, how deep (and low) you go is a choice you make. Odd as it may seem, the medical community does not really dispute the inference. According to them, when you are depressed the serotonin level in your brain drops.
Did you catch that?
The drop in serotonin is not the cause of depression, but a symptom; it comes as a result of being depressed, or in this case, choosing to refuse the cure when depression is in its earliest phase. In other words, depression is not chemically induced, but can be chemically sustained as the patient declines the cure. In cases where the chemical change is severe, it stands to reason that medicine(s) are sought to bring a brain back ‘chemically’ to where decisions can be made intelligently, but even then we should not discount God’s power to restore (that’s my testimony).
And even as they did not like to retain God in their knowledge, God gave them over to a debased mind, to do those things which are not fitting. Romans 1:28
If like Cain a person refuses to repent and do well, that one will be handed over to a debased mind, in this case depression. God is not mocked and we will reap what we have sown. God lets us go to attain that which we fervently pursue. We do not have to look very far to witness this. The world abounds with curses not imparted by God, but rather self-imposed by means of personal faithlessness. Historically, as Cain murders his brother, he fulfills the prophecy declared by God only one sentence before—sin reigns in the faithless heart.
He Dwells -- The B'log in My Eye 