Christians believe that Jesus Christ has ultimately overcome the power of evil in the world, but that evil remains until all things come under His rule. While always seeking an explanation in terms of the natural world first, it may be necessary to consider a spiritual cause in some cases of apparent mental illness, often spoken about in terms of 'demonic activity' or spiritual oppression.
This is a very sensitive issue, not least because for some people the idea of a spiritual cause is preferable to the notion that they are actually unwell and so they will actively seek deliverance ministry or an 'exorcism'. It goes without saying that offering the wrong sort of treatment in any situation is profoundly unhelpful, so it is important to work out what is required. In the Church of England each Diocese has a team of advisors, appointed by the Bishop, who have accumulated considerable experience in the area of deliverance ministry and in helping to discern between spiritual and psychological manifestations. They work in partnership with parish churches and can be contacted by your local vicar. There is no charge for this, and they can give advice on how best to progress.
Other Christian denominations may have different approaches, some of which may not take psychiatric or psychological factors into consideration so caution is advised.