Our adult Sunday School class continues in to study C.S. Lewis’s The Screwtape Letters. Yesterday, my co-teacher taught Letters 8 and 9, which were closely related. Screwtape began Letter 8 to Wormwood by saying, “Has no one told you about the law of Undulation?”

Undulation is ups and down. Think of a sine wave diagram; or, if you never took trigonometry, think of waves at a beach. Or, if you’re an inlander and have never spent much time at a large water body, think of the ripples that occur when you plunk a stone into a pond. Peaks and troughs go out from the source. At the ocean beach, waves of irregular size and duration crash to the shore. A sine wave diagram is a nice, clean, consistent show of ups and down: always the same height, always the same width. Boring, if you’re looking for adventure. The ocean waves are much more exciting.

Screwtape says that this cycle of undulation can be used against the humans. Wormwood can attack his “patient” during the troughs. In Letter 9, he explains how to do this. But, the senior devil says, God can also use the troughs. In fact, God uses them more than the peaks to see the Christian grow.

Screwtape explains that God could use his sovereign power to carry the human through, over, or around the trough. He could make it easy on the human. He kind of does a little of that when the person is a new convert. But as the person begins to mature in his Christian walk, God backs off. In fact, when the next trough hits, the Christian can feel abandoned by God. How does this help? Screwtape explains it this way, using his twisted, reverse logic.

He leaves the creature to stand up on its own legs—to carry out from the will alone duties which have lost all relish. …Our cause is never more in danger than when a human no longer desiring, but still intending, to do our Enemy’s will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obey.

That’s good stuff. Satan tempts us through the trough. But God gives us hidden strength to get through it, to continue in our devotion to him even though we can’t see him, can’t hear him, can’t find any evidence of his presence. In so doing, we are strengthened more than when we do the same thing during life’s peaks. Devotion is easy during those times and, while we are encouraged and enthusiastic, we do not necessarily grow in grace to the extent when we go through the trough and come out victorious.

I had more to say about this, but will end here. Possibly on Friday I’ll pick it up again.