Mental illness does not necessarily caused by demonic forces. It may be due to dietary deficiency ( Vitamins and minerals), mood fluctuation, bipolarity, severe financial breakdown, childhood abuse, nervous system disorder etc. A born again christian can be a victim of Psychological depression. As we go through the lives of Elijah, David, Job, Jonah we clearly find some instances of Psychological depression. Mere men as we are, it is common for us to come across with those experiences in our practical lives. Even Jesus' disciples cried out frantically when they were on the verge of drowning because of the sudden and terrible gale.
I would like to make emphatically clear that those who are born again Christians need not doubt their spiritual blessings even when undergoing those types of mental sickness. Even if I go through the valley of deepest darkness, Lord I will not fear, for your shepherd's rod and staff protect me. Psalm 23:4
I also believe that those who are born again Christians could be touched by the demonic forces until they are under the shelter of Jesus Christ. Jesus himself promised to keep them safe from the devil. Let the Holy Spirit empower every born again christian to over come all sorts of depression and mental illness.
the primary(elemental) cause of all evil things is spirits and forces of the "darkness", while elementary particles/units such as atoms, molecules, vitamins, biological cells, biological/body organs, brain, body systems, nervous system, etc. are physical creations of the true God as well as harmless structural particals/units, and fundamentally shouldn't, or to be more precise and direct, don't cause problems (by themselves), for example, John the Baptist had not eaten anything for most of the time (He only ate wild honey and locusts from time to time very rarely - only when he found some in the wilderness where he lived), but didn't suffer from any nutritional deficiency such as avitaminosis or hypovitaminosis
"sin"(a.k.a. "satan", "beast(666)", etc.) causes delusion(s), while "death" causes deterioration(s) such as vulnerability to things (such as high temperature, burning, freezing temperatures, injury, infection, trauma, etc.), diseases, illnesses, syndromes, mental disorders/deviations, depression, desolation, poverty, unfavorable rot/decay, unfavorable aging, etc. - if all such spirits are removed from the world, then there will no longer be any evil (thing) with any person in the world
Blessings