Is sickness ever part of God's will for believers?
When Adam and Eve gave in to Satan's temptation in the Garden of Eden, they brought both sin and evil into a formerly perfect world. The result of sin is death (Romans 6:23), and all of creation, including humanity, has been suffering under death's curse ever since (Romans 5:12; 8:19–22). Death encompasses both physical and spiritual death and means more than our hearts ceasing to function or spending eternity in hell. Among other things, death also includes the natural evils of our world—evils like sickness and disease. Although God allowed man to make the choice that enslaved him to sin, it was not God's will for humanity to sin—He is perfectly holy, His very nature being in exact opposition to sin (Psalm 5:4; 92:15; James 1:13). It is also not God's will for us to live forever with the consequences of sin. Not only has He provided Jesus as our Savior, one day He will create "a new heaven and a new earth" (Revelation 21:1) without the stain of sin. But even now God is sovereign over all. Despite the fact that sin and evil, with all their consequences, have taken hold of mankind, there is nothing that happens without God's allowance.
In most cases, illness and death are a natural outcome of the evil caused by man's sin, which affects our bodies and all of nature around us. However, there have been rare occurrences when God has directly inflicted an illness upon someone to teach a lesson or in cases of grave sin (Daniel 4:28–37; 2 Chronicles 26:19–20; Acts 5:1–10). He may also use sickness as a means of loving discipline for believers (Hebrews 12:5–11).
Regardless of whether God actively inflicts an illness or passively allows it, we can see His ultimate purposes: to bring glory to Himself and to reveal His nature to us. God's power and sovereignty allow Him to use even the direst of circumstances to bring about His sovereign will. This may include showing His power through miraculous healing (John 9:1–3), bringing non-believers to salvation, or any number of things—including those known only to Himself.
Make no mistake—God is grieved when those He loves are suffering. He often shows His love and compassion for us through others who minister to us and care for us during times of sickness (2 Corinthians 1:3–5). And no matter what happens, we can trust that God is good, because that is His nature. He will never unnecessarily afflict us, and He can use these times of sorrow to bring about good things in our lives or the lives of others (Romans 8:28).
Even though it can be difficult to experience illness or to watch those we love suffer, we can still have faith that God is sovereign and that He cares for each one of us. We may not see His plans and purposes come to fruition during our lifetimes, but we can rest assured that God knows what He is doing.