My beloved brother in Christ, I received your letter and saw the mental warfare you are going through with the evil spirit of sin, of fornication.
My brother, listen to a pauper in knowledge with a soul bereft of any good. When one struggles against sin with humility and the fear of God, with fervent spiritual work and with the guidance of an experienced spiritual father, God will never let him be lost. Only when he neglects his spiritual duties, and especially when he becomes proud, thinking that he is something, then it is possible for him to slip. But even then, if he seeks forgiveness with humility, he gets up again and is healed again. For the Lord's mercy is near to those of a contrite heart.
However, many trials occur for one to gain experience and spiritual wisdom, for without temptations it is impossible to obtain experience. Experience is not a skill derived from learning, but it is to have practical knowledge of the benefit and harm of temptations. If God does not let a person fall into temptations a little, it is impossible for him to become conscientious. When someone does something with good intentions, yet later it turns out that what he did was not right, God---since He observes the heart and tries the purpose for every deed---will bring things back to normal and will enlighten him to realize what he should have done. Infallibility belongs to God alone. No matter how perfect the saints were, they still had some blemishes. Therefore, they would go through temptations to obtain greater humility and caution, and to grow patient with their weak nature. A touch of anger, laughter, or idle talk does not take away a saint's holiness.
Only one must keep in mind not to despair. Even if one slips and sins ten thousand times a day, it is not justifiable in God's eyes for him to despair, but rather he should be hopeful and prepare for a fight, until God's mercy comes and delivers him.
There was once a monk who happened to slip and sin by himself continually, yet he would always arise at once and do his prayer rule. The demon that kept throwing him into sin lost his patience seeing the courage and hopefulness of this brother. So he visibly appeared to him, and said to him with vexation:
"Don't you fear God, you defiled wretch? You have just sinned, so with what face can you now stand before God? Aren't you afraid that God will burn you?"
But since this brother had a valiant soul, he said to the demon:
"This cell is a forge: you hit and get hit. As God is my witness, Who came to save the world, I will not stop fighting you, falling and getting up, beating and being beaten, until my final breath---and let's see who will win: you or Christ!"
When the demon heard this unexpected reply, he said:
"I won't fight you any more, because if I do, I'll make you win crowns."
Thenceforth, this brother was delivered from the warfare, and he sat in his cell weeping for his sins.
When God enlightens a person and he repents for his sins and walks with humility and caution, God does not let him be lost. Of course, his predispositions for his previous sins will become thorns and hindrances to him, but when he sees the waves rising menacingly, he should not despair, thinking that he will fall and be lost. On the contrary, he should hope in God with faith and struggle with humility under the spiritual guidance and training of his spiritual father, without fearing the violent storm that rose.