The Philokalia

by Nicodemus of the Holy Mountain

And the heart is then seen to be full, not of the divine Spirit and of godlike thoughts, but of evil. It is as the Lord said: 'The mouth expresses what fills the heart' (Matt. 12:34). For if the devil can induce the person he has taken possession of to utter what is harbored within, then that person will not merely call his brother 'dolt' or 'fool' but may well pass from insulting words to murder. It is in these ways that the devil fights against God and the commandment God gave about not being angry with one's brother without good cause. But the insulting words and their consequences could have been avoided had their initial provocations been expelled from the heart through prayer and attentiveness. Thus the devil achieves his purpose when he makes us break God's commandment by means of the thoughts that he insinuates into the heart.