John Piper, Desiring God, 1996, p. 222,
“I have never heard anyone say, “The really deep lessons of life have come through times of ease and comfort.” But I have heard strong saints say, “Every significant advance I have ever made in grasping the depths of God's love and growing deep with him, has come through suffering.” Samuel Rutherford said that when he was cast into the cellars of affliction, he remembered that the great King always kept his wine there. Charles Spurgeon said that those who dive in the sea of affliction bring up rare pearls.”
John Piper, Desiring God, 1996, p. 222,