And when all these things come upon you, the blessing and the curse, which I have set before you, and you call them to mind among all the nations where the LORD your God has driven you, and return to the LORD your God, you and your children, and obey his voice in all that I command you today, with all your heart and with all your soul, then the LORD your God will restore your fortunes and have compassion on you, and he will gather you again from all the peoples where the LORD your God has scattered you. (Deuteronomy 30:1-3 ESV)
Some persons, when they hear weeping, fasting, and ashes spoken of in various passages, and especially in Joel [ Joel 2:12], consider that repentance consists chiefly of fasting and weeping. This delusion of theirs must be removed. What is there said concerning the conversion of the entire heart to the Lord, and concerning the rending not of garments but of the heart, belongs properly to repentance. But weeping and fasting are not subjoined as perpetual or necessary effects of this, but have their special occasion. Because he had prophesied that the Jews were threatened with a very great disaster, he counseled them to forestall the wrath of God; not only by repenting, but also by manifesting their sorrow. For just as an accused man is wont to present himself as a suppliant with long beard, uncombed hair, and mourner’s clothing to move the judge to mercy; so it behooved them when arraigned before the judgment seat of God to beg, in their miserable condition, that his severity be averted. But although perhaps sackcloth and ashes better fitted those times, it is certain that there will be a very suitable use among us for weeping and fasting whenever the Lord seems to threaten us with any ruin or calamity. When he causes some danger to appear, he announces that he is ready and, after a manner, armed for revenge. Therefore, the prophet does well to exhort his people to weeping and fasting – that is, to the sorrow of accused persons, for he had just stated that their evil deeds were brought to trial. In like manner, the pastors of the church would not be doing ill today if, when they see ruin hanging over the necks of their people, they were to cry out to them to hasten to fasting and weeping; provided – and this is the principal point – they always urge with greater and more intent care and effort that “they should rend their hearts and not their garments” [ Joel 2:13]. There is no doubt whatsoever that fasting is not always closely connected with repentance, but is especially intended for times of calamity. Accordingly, Christ links it with mourning when he releases the apostles from need of it, until, deprived of his presence, they should be overwhelmed with grief [ Matthew 9:15]. I am speaking concerning a public fast, for the life of the godly ought to be tempered with frugality and sobriety that throughout its course a sort of perpetual fasting may appear. c But because that whole matter is to be investigated again where we discuss the discipline of the church, I now touch upon it rather sparingly.
Nevertheless, I shall insert this point here: when the term “repentance” is applied to this external profession, it is improperly diverted from its true meaning, which I have set forth. For it is not so much a turning to God as a confession of guilt, together with a beseeching of God to avert punishment and accusation.’ Thus, to “repent in sackcloth and ashes” [ Matthew 11:21; Luke 10:13] is only to evidence our self-displeasure when God is angry with us because of our grave offenses. Public, indeed, is this kind of confession, by which we, condemning ourselves before the angels and the world, anticipate the judgment of God. For Paul, rebuking the slothfulness of those who are indulgent toward their own sins, says: “If we judged ourselves…we should not be judged” by God [ 1 Corinthians 11:31]. Now, while it is not always necessary to make men open and conscious witnesses of our repentance, yet to confess to God privately is a part of true repentance that cannot be omitted. For there is nothing less reasonable than that God should forgive those sins in which we flatter ourselves, and which we hypocritically disguise lest he bring them to light.
Not only is it fitting to confess those sins which we commit daily, but graver offenses ought to draw us further and recall to our minds those which seem long since buried. David teaches us this by his example. For, touched with shame for his recent crime, he examines himself even to the time when he was in his mother’s womb, and acknowledges that even then he was corrupted and infected with the filthiness of the flesh [ Psalm 51:3-5]. And he does not do this to extenuate his guilt, as many hide themselves in a crowd and seek to go unpunished by involving others with them. David does far otherwise. He openly magnifies his guilt, confessing that, corrupted from his very infancy, he has not ceased to heap misdeeds upon misdeeds. Also, in another passage, he undertakes such an investigation of his past life as to implore God’s mercy for the sins of his youth [Psalm 15:7]. Surely then, at last, we shall prove that our drowsiness has been shaken from us, if we seek from God a release by groaning under our burden, by bewailing our evil deeds.
Moreover, we ought to note that the repentance which we are enjoined constantly to practice differs from that repentance which, as it were, arouses from death those who have either shamefully fallen or with unbridled vices cast themselves into sinning, or have thrown off God’s yoke by some sort of rebellion. For often Scripture, in exhorting to repentance, means by it a kind of passage and resurrection from death to life. And in referring to a people as having “repented,” it means that they have been converted from idol worship and other gross offenses. For this reason, Paul declares that he will mourn for those sinners who “have not repented of lewdness, fornication, and licentiousness” [ 2 Corinthians 12:21 p.]. We ought carefully to observe this distinction, lest when we hear that few are called to repentance we become careless, as if mortification of the flesh no longer concerned us. For the base desires that always pester us, and the vices that repeatedly sprout in us, do not allow us to slacken our concern for mortification. Therefore, the special repentance that is required only of certain ones whom the devil has wrenched from fear of God and entangled in deadly snares does not do away with the ordinary repentance to which corruption of nature compels us to give attention throughout our lives. (http://www.peacemakers.net/unity/jcrepentance.htm)