When sin entered the world through Adam, it had a continuous and worsening effect on mankind. It is then that death passed unto all men, and soon after the lifespans of ancient men began to decrease. Diseases also crept their way to the human body, and these are all part of the corrupting power of sin our lives.
These diseases affect both physical and mental health, and cause them to break down, until the person ceases to function properly or permanently. Mental illnesses in particular have clinical treatments through therapy or medication, but the Bible also offers some ways to handle such conditions.
If we only read the Bible carefully, we can see that it has a lot of solutions for some of the most common mental issues today. Depression, fear, anxieties, and other related issues can be handled more properly through the power of the Holy Spirit.