Martin Luther on becoming too sure of oneself:
For since the Corinthians, St. Paul’s own disciples, to whom he had also preached and imparted the unadulterated Word, fell so miserably, a striking example has been placed before our very noses, bringing home to us the statement of St. Paul in 1Cor. 10:12: “Therefore let anyone who thinks that he stands take heed lest he fall,” being too sure of himself. This prompts us, when seeing or hearing someone who has become such a sow and heathen, to say apprehensively and prayerfully: “Dear Lord, do not let me fall thus.” For this happens so easily when we feel secure and careless.
Martin Luther, Luther’s Works, Vol. 28: 1 Corinthians 7, 1 Corinthians 15, Lectures on 1 Timothy, ed. Jaroslav Jan Pelikan, Hilton C. Oswald and Helmut T. Lehmann, 1 Co 7:40 (Saint Louis: Concordia Publishing House, 1999).