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

They say that Abba Macarius the Egyptian [St. Macarius the Great] on one occasion went up from Scete to the Nitrian mountain, and as he drew near to a certain place, he said to his disciple, "Pass on a little in front of me"; and when he had done so there met him a certain heathen priest, who was running along and carrying some wood about the time of noon. And that brother cried out to him and said, "O minister to devils, where run thou?"  And the priest turned round and smote him with many severe blows, and he left him with but very little breath remaining in him, and he took up his wood and went on his way; and when he had gone on a little further the blessed Macarius met him on his journey, and said to him, "May you be helped, O man of labours?"  And the priest was astonished, and came to him and said, "What fair thing have you seen in me that you should salute me [in this gracious fashion] ?"  And the old man said to him, "I see that you toil, and that you do not know that you are toiling for naught"; then he said to the old man, "At your salutation I also was very sorry, and I learned that you did belong to the Great God. But a wicked monk met me just before you didst, and he cursed me, and I smote him even to death."  And the old man knew that it was his disciple [of whom he spake], and the priest laid hold upon the feet of Macarius, and said to him, "I will not let you [go] until you make me a monk"; and they came to the place where the brother was lying, and they carried him and brought him to the church of the mountain. Now when the fathers saw the heathen priest with him, they marvelled that he had been converted from the error which he had held; and Macarius took him and made him a monk, and through him many of the heathen became Christians. And Abba Macarius said, " ' An evil word makes wicked even those who are good, and a good word makes good even those who are wicked,' as it is written."

from Sayings of the Holy Desert Fathers

My friends, God does not ask or desire that man should mourn from sorrow of heart, but rather that, out of love for Him, he should rejoice with spiritual laughter. Remove the sin, and the tear of sorrow is superfluous for your eyes.

St. John Climacus

A sign of true repentance is the acknowledgment that we deserve all the afflictions, visible and invisible, that come upon us, and even greater ones.

St. John Climacus

Do not be surprised that you fall every day; do not give up, but stand your ground courageously. And assuredly, the angel who guards you will honour your patience. While a wound is still fresh and warm, it is easy to heal…

St. John Climacus

It is impossible for us who have fallen into the pit of iniquities ever to be drawn out of it, unless we sink into the abyss of the humility of the repentant.

St. John Climacus

But nothing causes such exceeding grief as when anyone, lying under the captivity of sin, calls to mind from where he has fallen, because he turned aside to carnal and earthly things, instead of directing his mind in the beautiful ways of the knowledge of God. So you find Adam concealing himself, when he knew that God was present and wishing to be hidden when called by God with that voice which wounded the soul of him yourself? Why are you concealed? Why do you avoid Him Whom you once longed to see? A guilty conscience is so burdensome that it punishes itself without a judge, and wishes for covering, and yet is bare before God.

St. Ambrose of Milan

A penitent is an undisgraced convict. Repentance is reconciliation with the Lord by the practice of good deeds contrary to the sins.

St. John Climacus

You came into the world to save sinners; therefore You came to save Me also? You came to find and to save him who was lost; therefore You came to seek me too, for I am one of the lost. O Lord, O my God and Creator! I should have come to You as a transgressor of Your law. I should have fallen at Your feet, cast myself down before You, humbly begging forgiveness, pleading with You and craving Your mercy. But You Yourself have come to me, wretched and good-for-nothing servant that I am; my Lord has come to me, His enemy and apostate; my Master has come and has bestowed his love of mankind upon me. Listen my soul: God has come to us.

St. Tikhon of Zadonsk

Repentance is the renewal of baptism. Repentance is a contract with God for a second life. 

St. John Climacus

Sinners that repent are still saved; both publicans and fornicators cleansed by repentance enter into the Kingdom of Heaven.

St. Tikhon of Zadonsk

If we neglect to repent, without any doubt, we will remain outside of the door.  Good deeds, natural in feeling, will in no way replace repentance.

St. Ignatius Brianchaninov

By resolving to make one's confession, the soul is thereby held from sinning as by a bridle. For what we do not confess, that we do fearlessly as though in the dark.

St. John Climacus

Christ prayed for those that crucified Him: ‘Father, count not this sin against them; they know not what they do.’ Archdeacon Stephen prayed for those who stoned him so that the Lord would not judge this sin against them. And so we, if we wish to retain grace, must pray for our enemies. If you do not find pity on a sinner who will suffer in flames, then you do not carry the grace of the Holy Spirit, but rather an evil spirit; and while you yet live, you must free yourself from his clutches through repentance.

St. Silouan the Athonite

Thanksgiving dulls the fierceness of your illness!  Thanksgiving brings the ailing person consolation!  The tutored heart, sweetened by thanksgiving, is renewed through the power of a living faith.  Illumined by the sudden light of faith, the mind begins to contemplate the divine Providence of God... The sickbed is often the place where one gains the knowledge of God and the knowledge of self.  The suffering of the body often brings spiritual joys, and the sickbed is often watered with tears of repentance and tears of joy in God.

St. Ignatius Brianchaninov

Be merciful to your soul in her infirmities; excessive strictness distracts from repentance and leads to depression and despair.

St. Ignatius Brianchaninov

In the words of the psalmist, "As you lie in bed, repent of what you say in your heart" (Ps. 4:4 LXX), that is, repent in the stillness of the night, remembering the lapses that occurred in the confusion of the day and disciplining yourself in hymns and spiritual songs (cf. Col. 3:16) – in other words, teaching yourself to persist in prayer and psalmody through attentive meditation on what you read. For the practice of the moral virtues is effectuated by meditating on what has happened during the day, so that during the stillness of the night we can become aware of the sins we have committed and can grieve over them.

St. Peter of Damascus

Repentance must be the soul of prayer, without which prayer is dead, stinking with proud and seductive conceit!  Repentance is the only door through which one can obtain from our Lord the saving pasture.  He who neglects repentance is bereft of all grace...  The merciful Lord prepared the way for our divine heavenly eternal Kingdom, showed the door through which we might enter into the salvic pasture of the Holy Spirit and Truth, the door of repentance.  If we neglect to repent, without any doubt, we will remain outside the door.  Good deeds, natural in feeling, will in no way replace repentance.

St. Ignatius Brianchaninov

Years are not needed for true repentance, and not days, but only an instant.

St. Ambrose of Optina

Have confidence in the compassion of our Creator. Reflect well on what you are now doing, and keep before you the things you have done. Lift up your eyes to the overflowing compassion of heaven, and while He waits for you, draw near in tears to our merciful Judge. Having before your mind that He is a Just Judge, do not take your sins lightly; and having also in mind that He is compassionate, do not despair. The God-Man gives man confidence before God.

St. Gregory the Great