Behold, thou art made whole: sin no more, lest a worse thing come unto thee. (John 5:14). Sin affects both one’s soul and one’s body. It is clear in some cases but less clear in other cases. The truth is that every bodily illness is caused by sin. You commit sins within your soul. They hurt your soul directly. Given that the life of your body depends on the soul, your body cannot be healthy when your soul is sick.
The very fact that sin brings darkness and gloom must have a disadvantageous effect on your blood, which is the basis of physical health. When you remind yourself of the fact that the sin separates you from God – the source of life – and puts you in opposition to all laws that affect both your own well-being and nature, you may be surprised how it is still possible for a sinner to stay alive.
That is possible solely thanks to God’s mercy, which waits for your repentance and conversion. Therefore, a sick person should hurry to cleanse their sins and reconcile with God before anything else. That will pave the road for effective medical treatment. I heard of a remarkable doctor who wouldn’t begin treatment until his patient confessed and took communion. The more severe the illness was, the more he insisted on it.
(St. Theophan the Recluse, 1815-94)