For in a contest there is much labor needed--and after the contest victory falls to some, to others disgrace. Is the palm ever given or the crown granted before the course is finished? ... Therefore no one can receive a reward, unless he has striven lawfully; nor is the victory a glorious one, unless the contest also has been toilsome.
-St. Ambrose of Milan, Chapter 15, Three Books on the Duties of the Clergy, 4th century


For I know, yea I know many, not merely fasting and making a display of it, but neglecting to fast, and yet wearing the masks of them that fast, and cloaking themselves with an excuse worse than their sin.
-St. John Chrysostom, Homily 20 on Matthew 6, 4th Century


Fight constantly with your thoughts and call them back when they wander away. God does not demand of those under obedience that their thoughts be totally undistracted when they pray. And do not lose heart when your thoughts are stolen away. Just remain calm, and constantly call your mind back.
-St. John Climacus, The Ladder of Divine Ascent, 4.101, p. 112, 6th century