Any Christian, regardless of denomination, knows that there are things in this world that can drastically alter the mind. Various things, such as head trauma, drugs, or even a single glass of beer, can alter a person's mental state. This simple concept, which is known to literally everyone, makes the extreme ignorance of mental illness completely inexcusable, especially to the Church.
But apparently, many Christians find a way to excuse the inexcusable. Far too many Christians delegitimise mental illness, either by blaming the victims, or by dismissing it altogether. Depression is a very common target. Sadly, depression does not discriminate. Your personal circumstances do not matter. You could have reason to be upset, but at the same time, you might have no reason at all. You could be wealthy, well educated, loved by many, and you may well stay like that for the foreseeable future, but depression can still affect you. How? For the same reason you might still have a heart attack, or get lung cancer, or be attacked by a criminal, or get in a car crash etc.
Depression, and indeed any other mental illness, transcends personal circumstances. It is not, nor has it ever been, a case of positive thinking, or simply having faith. Faith alone doesn't heal a single wound or disease. Sure, you can pray, and if God wills (1 John 5:14), He might heal you, just as He is perfectly capable of healing other things. But there is no guarantee, and it is positively devastating to someone who has depression if you tell them otherwise.
There is one difference between mental illness and other problems. The brain, of course, is unique. When something affects other organs, the sufferer can usually think clearly, and even maintain a positive attitude. But that which affects the brain affects one's ability to think. Just as alcohol alters one's thought process, so does mental illness of any kind. But it doesn't wear off, and unlike other mind-altering things, such as drunkenness, it isn't as avoidable as not picking up a drink.
Now, as Christians, we are called to care for the sick. Granted, we're not all called to be doctors, but we are all called to love one another (e.g. John 13:34-35) and bear one another's burdens (Galatians 6:2). This makes it essential to both recognise that mental illness exists, and care for those affected.
Unfortunately, it is impractical to learn everything about mental illness. Indeed, even doctors do not know everything. But ignorance is no excuse. Indeed, it only raises our responsibilities. Belittling sufferers, or downplaying mental illness, is evil, and may lead to greater harm. We have two choices as Christians: Support those with mental illness, at least as far as we are equipped to do so, or simply abstain from talking about it. Woe betide the fools who act like it doesn't exist, and woe betide the devils who blame the victims.