Saint Nikolai Velimirovici,
You ask me if the mental sins are dangerous. As a monk you know it well. You know that a holy man said that the essence of monarchism consists in purifying the mind of bad thoughts. You know that the Church takes into account three kinds of sins: by deed, by word and by mind. That’s why we pray to the Father of Light for the reposed, to forgive them all their sins, done by deed, word and mind. And the fact that God uncovers the sinful thoughts is found in the Gospel: `And seeing Jesus their thoughts said: Why do you have bad thoughts in your hearts?`
Satan did not sin in another way than by his proud thoughts. For this reason he was banished from God and thrown into hell. The bad thoughts are the seed of all evil. From this seed grow the sinful words, desires and deeds. Remember another parable of Christ, the one about the sower: A man went out to sow his land. And when the men were sleeping their enemy came and sowed tares among the wheat. Gos sows good thoughts in the soul of everyone. If someone is idle and doesn’t take care of his soul as if it were a sowed land he is like sleeping. And while he is sleeping the evil spirit, the enemy of God and men comes and sows tares in the soul, videlicet bad thoughts. And from the bad thoughts to the bad words and deeds is not such a big distance, it’s like from the seed to the root.
Meaning that there is no distance between them and they all stay connected organically. Thus watch yourself. Close your eyes more often and as Nikita Stethatos says: watch the thoughts that float on the sea of the mind. The main strife in monarchism is to uproot the bad thoughts before they grow big and take hold of the soul and get into action. Crush them on the stone. As the psalmist says: `Wretched daughter of Babylon, blessed is the one who takes and crush your babies on the stone!` Do you understand the spiritual meaning of these words? Babylon is the kingdom of the devil and his children are the bad thoughts. The stone is Christ. Thus blessed is the one who crushes from the beginning the evil on the eternal stone, Christ. Since we both know this thing there is no other thing left than do exactly what should be done. Rejoice in God!
Source: Moldova Ortodox