Strictly speaking, a "penitential" is a libellus (small book) designed for pastoral use covering every kind of misbehavior that Christians consider "sinful" (i.e., offensive to God in contrast to a breach of legal requirements), arranged within a specific theological framework, specifying detailed amounts of penance as remedies. In this sense few penitentials with Irish links have survived: four in Latin (those of Finnian (6th century); Columbanus (6th-7th century); Cummean (7th century); and the Bigotian (8th century)) and one in Irish (before late 8th century). It is clear from surviving texts that these are only a fraction of the number that were compiled or used in Ireland. The term is, however, applied more widely to cover a range of early medieval legal texts which make prescriptions, regarding sinful acts, using the pattern found in penitential libelli (e.g., the Canones Hibernenses). The term is also used more loosely for the system of Christian penance, usually with the gloss that it emerged in Ireland, which was used in the West between the disappearance of "public penance" and the appearance of individual "confession."
By the fifth century, Latin Christianity had developed a practice with regard to "sins committed after baptism" known as "public penance" (admission of the faults to the bishop followed by public separation within the community) which applied to the "greater sins:" murder, apostasy, and fornication. This practice was a failure. And, that failure was compounded by the theological justifications made in its defense (e.g., by Jerome and Augustine) that it was a "laborious baptism" available only once in the Christian’s life and was to be truly difficult. The practice invoked a notion of sin as a crime deserving divine retribution where "doing the penance" was simply the sinner applying this punishment to themself. While several writers (e.g., Caesarius of Arles (c. 470-542)) pointed out that the whole system was a pastoral disaster, such voices went unheeded for fear of breaking faith with the past and its eminent supporters. Moreover, the system did not take account of the everyday sins, nor link the notion of penance for sins with the "doing of penitence" (cf. Mt 3:2 as found in Latin) preached as a basic part of Christian living.
Where in the British Isles the break with that practice was made is not clear (some of the earliest penitential-like texts have titles that link them to sub-Roman Britain: e.g., the "Synod of the Grove of Victory," and such legislation supposes the theoretical understanding that only a full penitential could supply), but the oldest extant formal penitential is by Finnian. The peniten-tials present a new view of (1) a sinful offense’s nature, (2) of the purpose of doing penance, and, (3) a new understanding of religious culpability. In contrast to the notion of a crime demanding a punishment—an assumption in Roman law—they adopt a notion of crime that closely resembles the system of debts found in Brehon Law whereby a crime, for example, homicide, produced a debt for the murderer to the dead person’s family which had to be repaid, and the size of the fine varied with the gravity of the action, the status of the offender and the offended, and the intention of the offender. Thus any sinful act’s penitential "loading" depended on the action (e.g., homicide is worse than theft), the actor (e.g., a cleric is more culpable), the one offended—if this is applicable (e.g., stealing from a church is worse than other thefts), and with what intention (e.g., by accident or neglect, or in hot temper, or cold-bloodedly). So just as a crime against another person produced a debt, so a crime against God produced a debt that could be worked-off (the system inherently allowed for the repetition) with suitable religious payments of prayer, fasting, and alms (cf. Mt 6:2-18). The other key element in the penitentials’ understanding of sinfulness is that penance is not seen as retribution, but therapy; while sin is viewed as a symptom of sickness rather than a manifestation of evil. This derives from John Cassian (c. 360-435) whose writings form the basis of western monasticism. Cassian saw sinful acts as expressions of eight underlying vices (called "principal" as they are the principia (sources) from which sins flow) imagined as chronic illnesses deep within individuals and requiring suitable medicines prescribed by a physician. The monastery was the place where these received chronic therapy following a dominant assumption of late antique medicine: "contraries heal contraries"—just as, for instance, the physical illness of fever needs cold, so the spiritual illness of gluttony requires fasting. Thus in extant libelli even when Cassian is not quoted, medical language is applied to the reconciliation process, and the sins are arranged systematically under the vices which produce them. Since the actual practice depends on this theological underpinning, legal texts with penitential-like materials should not be seen as proto-penitentials, as often happens, but as legal supplements to an established system of penitence based in the use of penitentials.
The penitentials’ originality lay in extending to everyone and every action, however serious, a method for helping monks overcome ongoing imperfections. This avoided the problems of public penance in being repeatable and linking penance for sins with everyday penitential practice. There is no evidence that new discipline met resistance in Ireland, but the system did encounter some resistance among Anglo-Saxon clergy, and later much sterner opposition among Frankish clergy. However, the practice gradually gained ground probably due to its pastoral practicality, and left a complex legacy to the Western church: it generated an increasing awareness of the place of internal contrition and conscience in sin, it led directly to the development of indulgences whereby one penance was replaced by another of equal worth but with less physical demands, and it provided the practical—and some of the theological—background to all later Western systems of penance (e.g., what the twelfth-century canonists called the "sacrament of penance").
