“Blessed are the pure in heart, for they shall see God,” since purity of heart leads to perfection. Two things are contained within the heart—goodness which is natural to it and evil which is unnatural. This latter gives rise to such passions of the soul as murmuring, envy, detraction, and all the rest.
Goodness, on the other hand, promotes knowledge of God and rids the soul of all these passions. If people honestly try to root out vice and avoid evil, if they repent with tears and sighs, devoting themselves humbly to a life of prayer, fasting, and watching, the Lord in his goodness will come to their aid and free them from all sinful inclinations.
Many who have lived a celibate monastic life for a long time have failed to learn what purity of heart is, because instead of studying the teaching of the fathers, they have followed their own wayward desires. So evil spirits and rebel marauders of the air have prevailed against them, hurling invisible darts by day and night, and thus preventing them from finding rest anywhere. Moreover they fill their hearts with pride, vanity, jealousy, criticism, raging anger, strife, and any number of other passions.
My only reason for writing you this letter is my desire for your salvation. I want you to be free and faithful and pure brides of Christ, the Bridegroom of all holy souls; as Saint Paul says: “I have espoused you to one husband that I may present you as a chaste bride to Christ.”
Let us awake, then, while we are still in this body, and grieve overourselves, lamenting day and night from the bottom of our hearts, so that we may escape the bitter torment, the weeping, wailing, and remorse that will have no end.
We must beware of entering through the wide gate and taking the easy road that leads to perdition, for many go that way. Instead we must enter by the narrow gate and take the path of sorrow and affliction that leads to life. Few people enter this gate, but those who do are real workers who will have the joy of receiving the reward of their labors and will inherit the kingdom.
If any are prepared to set out I do beg them not to delay and waste time, for they may be like the foolish virgins and find no one willing to sell them oil. These virgins burst into tears and cried out: “Lord, open to us.” But he answered: “Truly I say to you, I know you not.” And this happened to them simply because of their laziness.
I beg you by the grace of God to obey me as I also will obey you; and may we all obey the Lord who said by the tongue of the Prophet: “Who longs for life and desires to see good days? Keep your tongue from evil talk and your lips from deceitful speech. Turn away from evil and do good; seek and strive after peace.”
Letter 20: PG 40, 1056-1058 1061