I have been reading The Screwtape Letters by Lewis this week with my students. In an early part of the book, Screwtape (a devil character) writes about what he calls the Law of Undulation. This is the ordinary way God sanctifies His people, through ups and downs, peaks and valleys. Screwtape writes that it is necessary to keep humans unaware of this cycle in order to always catch them off guard, not expecting anything. But the faithful and mature know that God is a faithful Father who disciplines the sons and daughters that He loves. The weak and unbelieving arrive at the bottom of the valley surprised, dumbfounded, doubting their families, their churches, and their savior. Whereas the Christian responds in faith and joy, knowing that this is the way of holiness, the Law of Undulation. I would highly recommend the book if you have never read it before, and even if you have.