Simply put, self-discipline is the willingness to subordinate personal desires and objectives to those that are selfless and divine, to subordinate that which is attractive and easy to that which is right and necessary. For the Christian, self-discipline is obedience to the Word of God, the willingness to subordinate everything in our lives—physical, emotional, social, intellectual, moral, and spiritual—to God’s will and control, and for God’s glory.
MacArthur, J. F., Jr. (1991). Romans. Chicago: Moody Press.