Sin, to one who loves God, is nothing other than an arrow from the enemy in battle. The true Christian is a warrior fighting his way through the regiments of the unseen enemy to his heavenly homeland.
Unless curbed by the fear of God that accompanies the practice of the virtues, spiritual knowledge leads to vanity; for it encourages the person puffed up by it to regard as his own what has merely been lent to him, and to use his borrowed intelligence to win praise for himself. But when his practice of the virtues increases concomitantly with his longing for God, and he does not arrogate to himself more spiritual knowledge than is needed for the task in hand, then he is made humble, reduced to himself by principles which are beyond his capacity.
If however any one thinks that he is not being burned when sinning, to him the Scripture saith, Shall a man wrap up fire in his bosom, and not burn his clothes? For sin burns the sinews of the soul, and breaks the spiritual bones of the mind, and darkens the light of the heart.
Those who have sinned must not despair. Let that never be. For we are condemned not for the multitude of evils, but because we do not want to repent...
It is vain that some unenlightened people seek the greatest evil for man somewhere else, rather than in sin. Some consider disease to be the greatest evil, others - poverty, and others - death. But neither disease, nor poverty, nor death, nor any other earthly disaster can be such a great evil for us as sin is. These earthly misfortunes do not separate us from God if we are seeking Him sincerely, but, on the contrary, they bring us closer to Him.
The only thing God requires of us is that we do not sin. But this is achieved, not by acting according to the law, but by carefully guarding the divine image in us and our supernal dignity. When we thus live in our natural state, wearing the resplendent robe of the Spirit, we dwell in God and God dwells in us. Then we are called gods by adoption and sons of God, sealed by the light of the knowledge of God.
Woe is he who knowingly chooses to sin with the intention to repent when morning comes, for he knows not what the coming day or the night that precedes it will bring.
There is a sin which is always 'unto death' [1 Jn 5:16]; the sin which we have not repented. Even a saint's prayers will not be heard for the unrepented sin. The person who repents correctly does not imagine that his sins are cancelled through his own effort; but knows that through this effort he makes peace with God.