Question: "Is it sometimes God's will for believers to be sick?"
Answer: The biblical doctrine of the sovereignty of God states that God is almighty over all. He is in complete control of all things—past, present and future—and nothing happens that is out of His jurisdiction. Either He directly causes—or He passively allows—everything that happens. But allowing something to happen and causing something to happen are two different things. For example, God caused the creation of the perfect, sinless Adam and Eve; then He allowed them to rebel against Him. He did not cause them to sin, and He certainly could have stopped them, but He chose not to for His own purposes and to bring about His perfect plan. That rebellion brought about all manner of evil, evil that was not caused by God but which was allowed by Him to exist.
Sickness is one manifestation of the two broad types of evil—moral and natural. Moral evil is man's inhumanity to man. Natural evil is composed of things like natural disasters and physical sickness. Evil itself is a perversion or corruption of something that was originally good, but is now missing something. In the case of sickness, illness is a state where good health is missing. The Greek word for evil, ponerous, actually implies a malignancy, something that is corrupting a good and healthy state of being.
When Adam sinned, he condemned all of humanity to suffer the consequences of that sin, one of which is sickness. Romans 8:20-22 says, “For the creation was subjected to frustration, not by its own choice, but by the will of the one who subjected it, in hope that the creation itself will be liberated from its bondage to decay and brought into the glorious freedom of the children of God. We know that the whole creation has been groaning as in the pains of childbirth right up to the present time.” God—the “one who subjected” the creation to frustration following the Fall—has a plan to eventually liberate creation from its bondage to sin, just as He liberates us from that bondage through Christ.
Until that day, God uses sickness and other evils to bring about His sovereign purpose, to glorify Himself, and to exalt His holy name. At times, He miraculously heals sickness. Jesus went through Israel healing all manner of sickness and disease (Matthew 4:23) and even raised Lazurus from the dead after illness killed him. At other times, God uses sickness as a method of discipline or as a judgment against sin. King Uzziah in the Old Testament was struck with leprosy (2 Chronicles 26:19-20). Nebuchadnezzar was driven to madness by God until he came to understand that “the Most High rules in the affairs of men” (Daniel 4). Herod was struck down and eaten by worms because he took God’s glory upon himself (Acts 12:21-23). There is even at least one case where God allowed disease—blindness—not as punishment for sin, but to reveal Himself and His mighty works through that blindness (John 9:1-3).
When illness does come, it may not be the result of God’s direct intervention in our lives, but is rather the result of the fallen world, fallen bodies, and poor health and lifestyle choices. And although there are scriptural indicators that God wants us to be in good health, (3 John 2), all sickness and disease are allowed by Him for His purposes, whether we understand them or not.
Sickness is certainly the result of the fall of man into sin, but God is very much in control, and He does indeed determine how far evil can go (just as He did with Satan and Job's trials—Satan was not allowed to exceed those boundaries). He tells us He is all-powerful over fifty times in the Bible, and it is amazing to see how His sovereignty unites with the choices we make (both bad and good) to work out His perfect plan (Romans 8:28).
For those who are believers and suffering with sickness, illness, and/or disease in this life, the knowledge that they can glorify God through their suffering tempers the uncertainty as to why He has allowed it, something they may not truly understand until they stand in His presence in eternity. At that time, all questions will be answered, or perhaps more accurately, we will no longer care about the questions themselves.