I believe that until we reach heaven we will never know why God heals one person and not another. What we do know for sure is that God is on the throne; He is in absolute control of all things and will always do what is right. Rom. 8:28 tells us that "in all things God works for the good of those who love him"so we can be sure of God's love and the fact that if we are not healed then it is not because He does not care about us. He will always do what is best for His kingdom and our eternal good. An example of God healing for the good of His kingdom is in John 9:1-3 where Jesus healed a man who had been born blind. The disciples asked if sin was the cause of the man's blindness and Jesus replied, "Neither this man nor his parents sinned, but this happened so that the work of God might be displayed in his life." This man's healing was used by God to further the kingdom. Also, Job, a righteous man, suffered at the devil's hand even though he had not sinned. God has given us Job's story to make us aware of activities in the spirit world which we know little about. Who knows, some of us may face trials similar to Job's?
Sin related illness
Sin was not the cause of the man's blindness in John 9:1-3 but sin can cause health issues as John 5:1-15 tell us: After Jesus healed a man who had been an invalid for thirty-eight years, He told him in verse 15, "Stop sinning or something worse may happen to you." This story tells us that sin can be behind sickness and this is also shown in James 5:14-16 which say, "Is any one of you sick? He should call the elders of the church to pray over him and anoint him with oil in the name of the Lord. And the prayer offered in faith will make the sick person well; the Lord will raise him up. If he has sinned, he will be forgiven. Therefore confess your sins to each other and pray for each other so that you may be healed." These verses make it quite obvious that sin can cause illness and also that healing can come from confessing our sins. The Psalms also show a connection between sin and sickness. Psalm 38:3 says, "Because of your wrath there is no health in my body; my bones have no soundness because of my sin."Psalm 32:1-5 and Psalm 103:1-5 show this as well.
I have heard stories of people being physically healed when they were first saved and this is backed up by what is said in Mark 16:15-18. These verses are in the context of evangelists going into the world and preaching the Gospel. Speaking to the disciples, Jesus said, "Go into all the world and preach the good news to all creation. Whoever believes and is baptised will be saved ... they will place their hands on sick people, and they will get well." So, sick people may receive both spiritual and physical healing when they first believe.
Death is inevitable
When Adam sinned, physical death entered the world and affected all mankind so we can't expect to be healed of every disease and live forever. Common sense tells us that most of us will die from some illness so we should not always expect to be healed.
So, the Bible tells us that illness may -
be from God so that He can be glorified through healing us.
be used by God to try us like with Job.
be caused by sin and healed by confession.
be used to discipline us as shown in the Psalms.
be a natural part of life that we must accept.
Other than the promised healing from a sin related sickness, as seen above in James 5:14-16, a Christian has no biblical guarantee of being healed so it is wise to be willing to accept that fact.
Some Christians say that God does not want us to ever get sick or that He will heal us of any and every sickness but that is not scriptural (more on this below). Some even say that if a person is not healed then it is because of sin or weak faith. That is a very judgemental, hurtful and ungodly thing to say and it may destroy a person's faith. Also, it implies that God is a God of the strong and does not care for the weak. When I think of this it seems to me that God would be more likely to heal the weak to strengthen their faith and allow the strong to stand fast in sickness to show their faith. That is just a thought, who but God knows the truth of each individual case?
Healing in the atonement
1 Pet. 2:24-25 are often used to support healing but these verses are not talking about physical healing, rather they are talking about how our sinful souls were spiritually healed: "He himself bore our sins in his body on the tree, so that we might die to sins and live for righteousness; by his wounds you have been healed. For you were like sheep going astray, but now you have returned to the Shepherd and Overseer of your souls." These verses are in the context of sin, in past tense, and are about our salvation. In plain words Peter is saying, "You were lost and sin-sick sheep but now, by Jesus' wounds on the cross, you have been healed, you have been saved, and have returned to the Shepherd of your soul." Returning to our Shepherd can only be understood in the sense of sin because it is sin that separates us from God, not sickness. So it can be seen that the healing spoken of in these verses is not physical but spiritual healing which comes from the forgiveness of sin through Jesus' death on the cross.
Matt. 8:16-17 are also used to support healing in the atonement but these verses are only telling us that Jesus fulfilled the Isa. 53:4 prophecy about Himself: "...he drove our the spirits with a word and healed all the sick. This was to fulfill what was spoken through the prophet Isaiah: 'He took up our infirmities and carried our diseases.'" These verses only tell us that Jesus fulfilled the prophecy that He would heal; there is no mention of healing for all believers. If there really was healing in the atonement then it would be for everyone; we would all be healed all the time and live forever. Also, all new believers who have the faith to be saved and cleansed of all sin would be immediately healed of all their diseases by that same faith - the blind would see, the lame would walk etc. but we know that does not happen. If a blind man was saved and not healed would we tell him that he has enough faith to be saved but not enough faith to be healed? There is a guaranteed spiritual healing for those who put their trust in Jesus but there is no guarantee of physical healing even though it occurs at times according to God's will.
Physical sickness is a reality of life and, putting aside unnatural events, we are all going to die from some health problem. I believe that we should confess our sins and pray for healing and for the faith to gracefully accept whatever God chooses to do with our life. We should never consider for a moment that He does not love us or that weak faith is preventing our healing. In Luke 22:42, Jesus faced His suffering saying, "Father, if you are willing, take this cup from me; yet not my will, but yours be done." So, let's stand strong in our faith like Jesus and remember that many souls have been saved through witnessing the graceful death of a saint ... it would be a wonderful thing to be used so mightily by the Lord at the end of our days.