It will not do to say that God only uses our cancer but does not design it. What God permits, he permits for a reason. And that reason is his design. If God foresees molecular developments becoming cancer, he can stop it or not. If he does not, he has a purpose. Since he is infinitely wise, it is right to call this purpose a design. Satan is real and causes many pleasures and pains. But he is not ultimate. So when he strikes Job with boils (Job 2:7), Job attributes it ultimately to God (2:10) and the inspired writer agrees: “They . . . comforted him for all the evil that the Lord had brought upon him” (Job 42:11). If you don’t believe your cancer is designed for you by God, you will waste it.
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UPDATE: Rhett Smith has written some posts in response to Piper, calling Piper "unreal" in the first and naming him as someone who might "simply throw Bible verses at someone" in the second. Big-time blogger Hugh Hewitt thought enough of Smith to link to them. If you visit, though, be warned: Smith doesn't seem to find much comfort in the idea that God might be present with someone and using their illness for a greater good.