During times of trials God prepares us in a way we are inclined to forget: He uses it to build godly character. When you walk along the sea shore, you notice that the rocks are sharp in the quiet coves, but polished in those places where the waves beat against them. God can use the “waves and billows” of life to polish us, if we let him.

Source: The Transformation Study Bible (NLT), Colorado USA: David Cook Publishers (2009), commentary p.1974