You ask if sins of the mind are dangerous? As a monk you know this best. You know that one of the holy fathers has said that the essence of monasticism is the cleansing of the mind of evil thoughts. You also know that the Church lists three types of sin: in deed, in word, and in thought. This is why we pray to the Father of light for the dead, that He would forgive them all sins, whether in deed, in word or in thought. And the fact that God knows the sinful thoughts, you read in the Gospel, “And Jesus, seeing their thoughts, said, ‘Why do ye think evil in your hearts?’”1 Satan did not sin in any other way except through evil thoughts. That is why he was cast out from before the face of the Lord and thrown into Hades.
Evil thoughts are the seed of every evil. From that seed sprout sinful words, sinful desires and sinful deeds. Remember Christ’s second parable about the sower. “A man sowed good seed in his field. And when people were asleep, their enemy came, and sowed tares among the wheat.”2 God sows good thoughts into the soul of every man. If someone gets lazy and does not keep watch over his soul like he would over a sown field, he is like the sleeper. And while he sleeps, the evil spirit comes, the enemy of God and man, and sows tares, which is to say evil thoughts, into the soul. And from evil thoughts to evil words, the distance is no greater than from a seed to a root of a plant. Which means, there is no distance, the two are organically connected.
So therefore, keep watch over yourself. Close your eyes more often, and as Saint Nikita Stifat said, “examine the thoughts sailing on the sea of the mind.”
In the rules of monasticism, the main exercise defined is the uprooting of evil thoughts—before they develop, grow, and rule over the soul, and finally turn into action. Dash them against the stone. As the Psalmist said, “O daughter of Babylon, who art to be destroyed, happy shall he be that taketh and dasheth thy little ones against the stones!”3 Do you understand the spiritual meaning of this? Babylon is the kingdom of the devil and his children are the thoughts. Christ is the Stone. Blessed is, therefore, the one who dashes the evil in himself from the start, destroying it with the eternal stone—Christ.
So, since you and I both know this, we have no other choice but to also act accordingly.
Rejoice in the Lord.