Only the gospel of Christ fully knows the mystery of sin and the problem of sin and everything which hides within it. The prodigal son of the Gospel is the perfect example of the repentant sinner. The Gospel shows us that man, through his free will, can share his life with Earth and with Heaven, with Satan and with God, with paradise and with hell. Sin gradually strips man of everything divine in him, paralyzes his every divine inclination and desire, until it finally throws him into the bosom of Satan. And then man reaches the plight of grazing the swine of his master, the Devil. The swine are passions, which are always greedy and gluttonous. In such a life, the unfortunate man is nothing more than insane. In a shocking parable of the Gospel, the Lord says about the prodigal son, ‘he came to himself,’ (Luke 15:17) How did he come to himself? He came to himself through repentance. Through sin, man becomes mad, insane. Every sin, even the most seemingly insignificant one, is always an insanity of the soul. Through repentance, man comes to his senses becomes complete again, comes to himself. Then he cries out loud to God, runs to Him, and cries towards Heaven, ‘Father, I have sinned against Heaven, and in thy sight’ (Luke 15:21). And what is the heavenly Father doing? He is always infinitely merciful upon seeing His child in a state of repentance. He has compassion for him, runs, embraces him, and kisses him. He orders His heavenly hosts, the holy angels: ‘Bring forth the best robe and put it on him; and put a ring on his hand, and shoes on his feet: and bring hither the fatted calf, and kill it; and let us eat, and be merry: for this is My son who was dead, and is alive again; and he was lost, and is found. And they began to be merry.’ (Luke 16:22-24) And this is taking place for each and every one of us, and for the sake of every sinner who repents. Namely, joy and happiness is taking place in the heaven of the All-merciful Lord and God, and together with Him, all of the holy angels.

St. Justin Martyr

Three old men once came to Abba Sisoes [St. Sisoes the Great] because they had heard that he was a great man. And the first one said unto him, "Father, how can I escape from the river of fire?" And Abba Sisoes answered him never a word. Then the second old man said to him, "Father, how can I escape from the gnashing of teeth, and from the worm which never dies?" And Abba Sisoes answered him never a word. Then the third old man said to him, "Father, what will I do? For the remembrance of the outer darkness troubles me." And Abba Sisoes answered and said to them, "I never think on any of these things, but I believe that God is Merciful, and that He will show mercy to me"; then the old men went away grieved at the answer which Abba Sisoes had spoken to them. Now because he did not wish to send them away sorrowful, he brought them back, and said to them, "Blessed are ye, O my brothers, for I have been jealous of you"; and they said to him, "In what matter have you been jealous of us?" And he said, "The first one of you spake about a river of fire; and the second spake about the gnashing of teeth and the worm which dies not; and the third spake about the outer darkness; if remembrances of this kind have dominion over your minds it is impossible for you to commit sin. What can I do who am stubborn of heart? For hardness of heart will not allow me to perceive even that there a punishment for men existeth, and because of this I sin every hour." And when the old men had heard these words, they made excuses to him, and said, "In very truth according to what we have heard, even so have we seen."

from Sayings of the Holy Desert Fathers

When the soul of a man departs from the body, a certain great mystery is there enacted. If a person is under the guilt of sin, bands of demons and fallen angels approach along with the powers of darkness which capture the soul and drag it as a captive to their place. No one should be surprised by this fact. For if, while a man lived in this life, he was subject to them and was their obedient slave, how much more, when he leaves this world, is he captured and controlled by them?

St. Macarius the Great

No one and not one thing can bring harm to a person if he does not harm himself, and conversely, a thousand means and ways to salvation will not help the person who will not avoid sin.

St. Nikon of Optina