The fear of death is actually a very valid fear, and with good reason. God created us for life everlasting, and therefore a healthy fear of death is in order. It’s a godly fear that awakens us to the reality of our Creator, and that we are indeed finite beings who are subject to Him. Unless an individual has a strong and biblically rooted faith in Christ, the fear of death is something most people experience at some level, to some degree.
God created every human being for life — with a strong will to live. Death was never God’s intention for us, and therefore it is contrary to our spiritual nature. We were not originally created to die, and therefore a certain fear of death is natural. We can only overcome death through Christ.
I believe most fear death because there is an association with suffering. Whether or not suffering is part of death or not, that is often the reason. However, I can say that death, in itself, is a natural process for the body and is painless.
Another reason is the uncertainty, and the fear of eternity. Again, we were not originally created to die. Therefore, we desire to hold onto the present, earthly, tangible, carnal life we have in our bodies. What we need to understand is that our full life is going to be realized in the spirit once we are with Christ in heaven. That is the ultimate liberty and life one can experience. And that is something He purchased for us. Apart from Christ, that life absolutely cannot and will not be experienced. And that is the death we need to truly fear, if any at all.
The biblical definition of death is to be eternally separated from God. I believe most people view death as something that happens physically. What we really need to understand is that true death, the death of the spirit (and not merely the body), is a spiritual and permanent experience that is full of eternal suffering and torment with the devil and his angels. Death is to exist apart from Jesus Christ.
That is the death we must fear. And for those who do not have Christ, that fear is legitimate and valid.