“God sets the limits of Satan’s power to cause pain. Our God is not frustrated by the power and subtlety of Satan. Satan cannot make a move without the permission of God Almighty. He may be a lion. But he is a LION ON A LEASH. And God reins him in or gives him slack according to God’s own sovereign purposes.”
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John Piper, Job: Reverent in Suffering
http://www.desiringgod.org/resource-library/sermons/job-reverent-in-suffering