Never despair in God's mercy by whatever sins you may have been bound by the temptation of the Devil, but pray with your whole heart, with the hope of forgiveness; knock at the door of God's mercy and it shall be opened unto you. I, a simple priest, am an example for you: however I may sometimes sin by the action of the Devil, for instance, by enmity towards a brother, whatever the cause may be, even though it may be a right cause, and I myself become thoroughly disturbed and set my brother against me, and unworthily celebrate the Holy Sacrament, not from wilful neglect, but by being myself unprepared, and by the action of the Devil; yet, after repentance, the Lord forgives all, and everything, especially after the worthy communion of the Holy Sacrament: I become white as snow, or as a wave of the sea, by the blood of Christ; the most heavenly peace dwells in my heart; it becomes light, so light, and I feel beatified. Then, indeed, I forget all troubles, anxieties, and the oppression of the enemy, I become entirely renewed, and as though risen from the dead. Do not then despair, brethren, whatever sins you may have committed, only repent find confess them with a contrite heart and humble spirit. Glory, O Lord, to Thy mercy! Glory, O Lord, to Thy long-suffering and forbearance!
Excerpts from the diary of St. John of Kronstadt on
Despair and Despair of Forgiveness
You feel straitened upon earth from all sides. Everything betrays you; your relations, friends, acquaintances, riches, the pleasures of the senses, your own body; all the elements — earth, water, fire, air, light — play you false. Cling, therefore, to God alone, "with Whom is no variableness, neither shadow of turning," Who alone is Love.
You, sinner, who have fallen into the depths of evil, when you represent to yourself the multitude of your sins and fall into despair and hardness of heart, remember that the heavenly Father sent His only begotten Son, our Lord Jesus Christ, into the world for your salvation from sins and from eternal condemnation for them. Turn then with faith to this Mediator before God for men, imploring Him from the depths of your soul to wash away by His all-cleansing Blood, shed for us on the cross, your iniquities too; turn zealously to repentance, confessing your sins before His priest as before Himself, that you may be justified, after which, if the minister of the sacrament of penitence finds you prepared and fit, draw nigh to the holy cup and you shall be cleansed of your sins: peace shall flow into your soul like a river, and you shall be the son of the heavenly Father, "who was dead and is alive again, and was lost and is found." 
Believe firmly that the wickedness of the Devil in you and your own wickedness will never conquer the unspeakable, infinite mercy of God. Great is the wickedness of the Devil in you, but the mercy of God is infinitely greater. Therefore, in times of doubt, incredulity, blasphemy, malice, envy, avarice, covetousness, involuntary hypocrisy, entreat the Lord with hope, and be sure that His infinite goodness will incline Him to have mercy upon you, if you turn from your wickedness.
When the darkness of the accursed one [the devil] covers you — doubt, despondency, despair, disturbance — then only call with your whole heart upon the sweetest name of Jesus Christ, and in Him you shall find all — light, strengthening, trust, comfort, and peace; in Him you shall find the greatest mercy, goodness and bountifulness; all these mercies you will find contained in His name alone, as though in a rich treasury.
If the Lord were not long-suffering, if He were not the Lover of men, would He have borne with our great offences? would He have been incarnate? would He have suffered and died for you? would He have given you His most pure Body and Blood, upon which even the angels look with fear and trembling? would He have saved you from sin and spiritual death so many innumerable times? Had it been otherwise He would have said: "Be tormented, if you are so evil by nature; I will not deliver you again after having delivered you so often before." But now, during all our life-time He bears with our innumerable offences, and still waits for our conversion. Glorify, then, His love and long-suffering. Picture to yourself what it would have been without Him, without Him to save? Horror and trembling fill the soul at the idea of it. But impenitent sinners will indeed be overtaken at the last by God's wrath "in the day of wrath and revelation of the righteous judgment of God." 
Unite your soul to God by means of hearty faith and you will be able to accomplish everything. Do powerful, invisible, ever-watchful enemies wage war against you? You will conquer them. Are these enemies visible, outward? You will conquer them also. Do passions rend you? You will overcome them. Are you crushed with sorrows? You will get over them. Have you fallen into despondency? You will obtain courage. With faith you will be able to conquer everything, and even the Kingdom of Heaven will be yours. Faith is the greatest blessing of the earthly life; it unites the man to God, and makes him strong and victorious through Him. "He that is joined unto the Lord is one Spirit." 
Who does not know how difficult it is, without God's special grace, for a sinner to turn from the way of sin that is so dear to him into the path of virtue? How deeply sin takes root in the heart of the sinner, and in all his being — how it gives the sinner its own way of looking at things, by means of which he sees them quite differently to what they are in reality, and shows him everything in a kind of alluring light. It is for this reason that we see that sinners very often do not even think of their conversion, and do not consider themselves to be great sinners, because their eyes are blinded by self-love and pride. And if they do consider themselves sinners, then they give themselves up to the most terrible despair, which overwhelms their mind with thick darkness and greatly hardens their heart.
When you pray that your sins may be forgiven, strengthen yourself always by faith, and trust in God's mercy, Who is ever ready to forgive our sins after sincere prayer, and fear lest despair should fall on your heart — that despair which declares itself by deep despondency and forced tears. What are your sins in comparison to God's mercy, whatever they be, if only you truly repent of them? But it often happens that when a man prays, he does not, in his heart, inwardly hope that his sins will be forgiven, counting them as though they were above God's mercy. Therefore, he certainly will not obtain forgiveness, even should he shed fountains of involuntary tears; and with a sorrowful, straitened heart he will depart from the Gracious God: which is only what he deserves. "Believe that ye receive them," says the Lord, "and ye shall have them." 
Even the saints of God were at times seized with diabolical despair and despondency. What, therefore, can we sinners expect? O, the enemy often wounds us by the wrath, humiliation, and cruel despondency of the heart! We must continually turn to God and be every moment with Him, in order that we may not be besieged with the wrath and despondency of the enemy.
There are other means besides of escaping from them — the broad way of the world; only if you give yourself up to worldly pleasures, despondency will leave you for a time, at least while the pleasures last. But afterwards you will be captivated by these pleasures; they will become a necessity to you, and you will find comfort and joy in them, and in them alone; but may God preserve every Christian from finding his only way of escape from the despondency of the Devil through such means! It is better to walk in the narrow path, to patiently bear despondency, and to seek frequent help and deliverance from the Lord Jesus Christ, Who always rejoices those labouring for their salvation for His sake, than to enter on the broad and smooth way of the world and purchase there, by means of the pleasures of the flesh, freedom from the spirit of despondency. By means of this spirit of despondency the enemy has driven many from the narrow path of salvation on to the broad, smooth path which leads to destruction.
You are praying, your prayer is successfully accomplished, and you have inward proof that the Lord hears it and deigns to receive it favourably; your thoughts are at peace, you feel lightness and sweetness in your heart; but at the end of your prayer, through some slight infirmity of your heart and thoughts, a heavy burden falls on your heart, a weakening fire, and you feel an extreme difficulty in praying, and even an aversion to it, instead of the former lightness and inclination to prayer. Do not despair, friend; these are only the snares of the enemy, who loves to mock at us, especially at the end of our pious employment, so that we may fall into despondency and consider all our previous labours in the holy work lost. Learn from this in future not to extinguish your spirit even for a moment during your prayer; pray in spirit and truth, unremittingly, and not flattering the Lord during prayer by one single word--that is, not pronouncing a single word feignedly, hypocritically; let your prayer be only the expression of truth, the mouthpiece of the Holy Spirit, and do not let it serve by a single word the enemy's lie, nor be in any way the organ of the Devil.
We must always remember that the Devil is continually trying to choke our soul with the dust of hell, of which there is already too much within us, and which is small and various; and thus, whether your heart be obscured by animosity, or impatience, or irritability, or by grudging your material possessions to your brother or yourself, that is avarice, or by love of gain, or love of money, or by the quarrelsome and offensive words of others, or by despondency and despair, or by envy, doubt, or incredulity, or by open unbelief, or by vanity, or by slothfulness in prayer or in any good work, and in general in the performance of your duty, say to yourself with firm assurance: "This is the dust of the Devil; this is the darkness of hell." By faith and hope in the Lord, by continual watchfulness over yourself, you will be able, with God's help, to escape from this dust and darkness. "He that is begotten of God keepeth himself, and that wicked one toucheth him not." 
When you stand praying, burdened with many sins and overpowered with despair, begin to pray with hope, with a fervent spirit, and remind yourself that "the Spirit Itself maketh intercession for us with groanings which cannot be uttered!"  When you remember with faith this action of the Spirit of God within us, then tears of emotion will flow from your eyes, you will feel in your soul peace, sweetness, justification, "and enjoy in the Holy Spirit,"  and you will cry in your heart, "Abba, Father!''
When you are praying, watch over yourself so that not only your outward man prays, but your inward one also. Though you be sinful beyond measure, still pray. Do not heed the Devil's provocation, craftiness, and despair, but overcome and conquer his wiles. Remember the abyss of the Saviour's mercy and love to mankind. The Devil will represent the Lord's face to you as terrible and unmerciful, rejecting your prayer and repentance; but remember the Saviour's own words, full of every hope and boldness for us: "Him that cometh to Me, I will in no wise cast out;"  and "Come unto Me, all ye that labour and are heavy laden" — with sins and iniquities, and the wiles and calumnies of the Devil — "and I will give you rest." 
O, how full of misery, of difficulties, and how grievous is this earthly life! From morning till night, daily, we must carry on a grievous warfare against the carnal passions, fighting against our soul, against "principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places,"  whose craft and subtlety are immeasurably evil, infernally artful, indefatigable! O, Sweetest Saviour, Thou Who calleth unto Thyself all those who labour and are heavy-laden — to give them rest! Thou seest: our heart and bosom are wasted from this daily struggle and affliction, we are unnerved, weakened, we walk like shadows. Our evil enemies continually vex our souls, and endeavour by every means to draw us into the abyss of despair. Stretch forth, Master, Thy mighty right hand, and deliver us from the snares of the dragon of olden time, the murderer of men. "If any man will come after Me, said the Lord, let him deny himself, and take up his cross daily and follow Me."  But who is the daily cause of our cross, our afflictions and distresses? The old carnal man, and the Devil with his continual snares.
Do not be despondent and do not fall into despair when you feel within your soul the deadly breath and ferment of malice and evil, impatience and blasphemy, or any weakness from impure thoughts; but fight against them unremittingly and endure valiantly, calling with all your heart upon the Lord Jesus — the Conqueror of hell. Humble yourself deeply, deeply, acknowledging yourself from the depths of your soul as the first of sinners, unworthy of human fellowship, and the Lord, seeing your humility and your struggle, will help you. Call also to your help the speedy Mediatrix, the Most Holy Virgin, the Mother of God, saying: " Heal, most pure Lady, the many painful wounds of my soul, and strike the enemies constantly fighting against me."
How many times already, O Master, Lord Jesus, hast Thou renewed my nature, heedlessly corrupted by my sins! There is no measure and number to this. How many times hast Thou saved me from the furnace burning within me, from the furnace of many and diverse passions, from the abyss of despondency and despair! How many times hast Thou renewed my depraved heart, when only I have called upon Thy Name with faith! How many times hast Thou accomplished this through the life-giving Holy Sacrament! O Lord! in truth there is no number and measure of Thy mercies unto me, a sinner. What shall I offer to Thee, or what can I render unto Thee for Thine innumerable benefits to me, Jesus, my life and my lightness? May I be prudent in my ways, according to Thy grace; for " blessed are those that are undefiled in the way,"  as Thou hast said, through the Holy Spirit, by the mouth of our forefather, David. I will endeavour to be faithful unto Thee, to be humble, meek, not irritable, gentle, forbearing, industrious, merciful, generous, not covetous, obedient.
When you sin for the fiftieth and hundredth time in the day, and are seized with the most devilish despondency, and despair in God's mercy, say, from the depths of your soul, with Metaphrastes: "I know well, O Lord, that mine iniquities have gone over my head; but I also know that without measure is the multitude of Thy bounties, and unspeakable is the mercy of Thy great goodness, and no sin is there that can overcome Thy loving-kindness. Therefore, O most wonderful King, O Lord most good, do Thou show Thy mercies in me, a sinner; manifest in me the power of Thy goodness and the might of Thy loving-kindness, and receive me who turn to Thee. Accept me as Thou didst the prodigal, the thief, and the sinful woman. Accept me, though in word and in deed, by my evil passions and unreasonable imaginations, I have sinned without measure against Thee. But, O Lord, O Lord, rebuke me not in Thy righteous wrath; neither chasten me in Thy hot displeasure. Have mercy upon me, O Lord; for, though I am weak, I am also Thy creature. Thou, O Lord, hast established Thy fear in me, and yet I have done evil in Thy sight. O Lord my God, I have put my trust in Thee. If there is any hope of salvation for me, if Thy loving mercy can overcome the multitude of my transgressions, be Thou my Saviour, and, according to Thy goodness and mercy, loosen, remit, and forgive all wherein I have sinned; for my soul is full of trouble and there is no hope of salvation in me. Have mercy upon me, O God, according to Thy loving-kindness; deal not with me according to my sins: but turn, preserve, and deliver my soul from the evils besetting it, and from all its wicked undertakings. Save me for Thy mercy's sake, that where sin abounded Thy grace may much more abound, and I will glorify Thee always, all the days of my life. For Thou art the God of the penitent, and the Saviour of sinners." 
Be bold, resolute in every good work, be especially generous in words of kindness, tenderness, sympathy, and still more so in works of compassion and mutual help. Consider despondency, despair in any good work, as an illusion. Say: " I can do all things through Christ Which strengtheneth me,"  though indeed I am the greatest of sinners. " All things are possible to him that believeth." 
ou thus learn by experience that every earthly enjoyment means vanity and vexation of spirit, and that without God, in spite of all comforts tending to an outwardly happy life, we are but poor, miserable creatures; that to have Christ in our hearts is to possess a rich, beautiful, bright dwelling-place, every adornment, peace, and comfort; and thus bear your sorrow patiently, and steadfastly learn, with your whole heart, the lesson which the Lord teaches you through your affliction. Do not grow faint-hearted and do not despair in God's mercy. " A little affliction and then joy shall shine again. For the Lord is most merciful, and remembers that we are dust, that the days of a man are as grass, that he only flourishes like the flower of the field, which when the wind goeth over it is gone."  "And the Lord will not suffer us to be tempted above that we are able, but with the temptation will give us abundance of power, so that we may be able to bear it." 
 Romans ii. 5.
 1 Corinthians vi. 17.
 St. Mark xi. 24.
 1 John v. 18
 Romans viii. 26.
 Romans xiv. 17.
 St. John vi. 37.
 St. Matthew xi. 28.
 Ephesians vi. 12.
 St. Luke ix. 22.
 Canon to the Guardian Angel.
 James i. 17.
 Fourth Prayer before Communion: composed by St. Simeon the Metaphraste.
 Philippians iv. 13.
 St. Mark ix. 23.
 1 Corinthians x. 13.
 St. Luke xv. 24, 32.
Excerpts compiled from: My Life in Christ or Moments of Spiritual Serenity and Contemplation, of Reverent Feeling, of Earnest Self-Amendment, and of Peace in God, St. John of Kronstadt.