John Piper writes,

On July 2, [1505,] on the way home from law school, [Luther] was caught in a thunderstorm and was hurled to the ground by lightning. He cried out, “Help me, St. Anne; I will become a monk.” He feared for his soul and did not know how to find safety in the Gospel. So he took the next best thing, the monastery.

Fifteen days later, to his father’s dismay, he kept his vow. On July 17, 1505, he knocked at the gate of the Augustinian Hermits in Erfurt and asked the prior to accept him into the order. Later he said this choice was a flagrant sin—“not worth a farthing” because made against his father and out of fear. Then he added, “But how much good the merciful Lord has allowed to come of it!” We see this kind of merciful providence over and over again in the history of the church. We saw it powerfully in the life of Augustine, and we will see it in Calvin’s life too. It should protect us from the paralyzing effects of bad decisions in our past. God is not hindered in his sovereign designs from leading us, as he did Luther, out of blunders into fruitful lives of joy.

–John Piper, The Legacy of Sovereign Joy, pp.83-84.