J. C. Ryle

Job thought he knew his heart—but affliction came and he found he did not. David thought he knew his heart—but he learned by bitter experience how woefully he was mistaken. Peter thought he knew his heart, and in a short time he was repenting in tears. Oh, pray, beloved, if you love your souls, for some insight into your own corruption; the truest saints of God do never quite discover the exceeding sinfulness of that old man which is in them.

~ J.C. Ryle, A Bad Heart