Mental illnesses are diseases, not sins.
Today there are many people who suffer from depression, and there are many kinds of depression. Basically, one who suffers from depression is compromised to various degrees his/her ability to think and to act. Those who are the more serious, the catatonic and the clinically depressed, and those who experience serious chemical imbalance in their brain, should first consult medical intervention. When I meet with these cases, I would refrain from providing much spiritual guidance and exhortation, but simply offer to hold their hands and pray with them. They are not capable of making changes. They need medicine to help balance the chemicals in their brain first. Otherwise they would despair and even turn suicidal due to their inability to love God and make amends.
The less seriously depressed should also consult their doctors. Sometimes a light dose of serotonin-based medicine can make a huge difference, but sometimes not. However, grace builds on nature. If Medicine can help the brain to function better, the brain will faster recalibrate itself and the person can start acting on their will sooner.
When a person cannot think in a balanced way, either too euphoric or too depressed, his ability to engage in spirituality is often lopsided,with the depressed tending toward super-spiritualization; i.e., every wrong thing is God’s punishment for my sin, doctors and medicines cannot help because all my problems are spiritual ones, etc.
As their friends, we need to accompany them in this journey. Encourage and accompany them to see their doctors, make sure they take the necessary medications. Pray with them. Pray God’s healing upon them. Carry them when they want to give up and do not allow them to fall into despair. And if they have exhibited suicidal tendency, have them promise to call you whenever their thoughts take them there. Pray and fast for them. Fight their disease for them and love them as a beloved child of God, especially when they could not believe that, especially when they are the most difficult to love.
Remember, when they cannot pray, cannot trust God, cannot think anything good, when they turn cynical, when they blame God and themselves, when they blaspheme, it is not they who are doing this, it is their sickness. Pray, trust, hope, bless, and love on their behalf what they cannot do. Stand with them and stand for them before God. Do not give them up.