Here are some quotations written between the Fourth and Seventh Centuries by some powerful spiritual athletes.  They are from an amazing collection of writings — The Philokalia — Volume One.  Enjoy the quotes:

“When a man has an exact knowledge about the nature of thoughts, he recognizes those which are about to enter and defile him, troubling the intellect with distractions and making it lazy.  Those who recognize these evil thoughts for what they are remain undisturbed.”  –Isaiah the Solitary

“Whoever becomes angry or resentful is his own enemy.  He is like a man who wants to see clearly and yet inflicts damage on his own eyes.”  –Evagrios the Solitary

“We must take the utmost care to guard the heart from base thoughts.”  –John Cassian

“It is impossible for the mind not to be troubled by thoughts.  But if we exert ourselves it is within our power either to accept them and give them our attention, or to expel them.  Their coming is not within our power to control but their expulsion is.  The amending of our mind is also within the power of our choice and effort.”  –Abba Moses

“Journey without direction is wasted effort.”  –Mark the Ascetic

“Sin is a blazing fire.  The less fuel you give it, the faster it dies down; the more you feed it, the more it grows.”  –Mark the Ascetic

“Our intelligence should control our desires with wisdom and skill, regulating them, admonishing them, correcting them and ruling them as a king rules over his subjects.”  –Hesychios the Priest

“When the mind descends into the darkness of impure thoughts the passions drag it forcibly and against its will into their service.”  –Neilos the Ascetic

“Struggle first against external sins and then go on to struggle against evil thoughts and malicious words.”  –Diadochos of Photiki

“Through struggle we can reach a state in which our mind is no longer troubled.”  –John of Karpathos

“Regard as free not those whose status makes them outwardly free, but those who are free in their character and conduct.”  –Anthony the Great