Of the pilgrims' tales in The Canterbury Tales, the Parson's Tale stands out not only because it is the only tale in prose but also it signals a major shift from tales to entertain to religious instruction:
I wol you telle a myrie tale in prose/To knytte up al this feeste and make an ende. (Parson's Prologue, 10, ll. 46-7)
Even though the Parson promises to tell a merry tale to sum up and complete the cycle of tales told by the other pilgrim's, what he presents is a very serious and lengthy treatise on the way to heaven:
To shewe yow the wey, in this viage,/Of thilke parfit glorious pilgrymage/That highte Jerusalem celestial. (Parson's Prologue, 10, ll.49-51)
In other words, what follows will be a spiritual road map for the "perfect and glorious pilgrimage to the heavenly Jerusalem." Specifically, the Parson's "tale" in extended sermon on the key to salvation--repentance--and an exploration of the Seven Deadly Sins: pride, envy,anger, sloth, avarice, gluttony and lechery.
The Parson's chief purpose is to remind the pilgrim's that, just as they are on a pilgrimage to Canterbury, life is a pilgrimage to heaven or hell, and the Parson's "myrie" (merry) tale will show the pilgrims how to combat the deadly sins and reach their true home, "Jerusalem celestial."
The way to heaven, acccording to the Parson, is through penitence:
. . . this wey is cleped Penitence, of which man sholde gladly herknan and enquere with al his herte,/to wyten what is Penitence. . . .(Parson's Tale, 10, ll.81-83)
In other words, man should study penitence in all its forms in order to understand its nature. Penitence becomes important to salvation because it provides the method by which sinners can obtain forgiveness of their sins and reach heaven.
Much of the rest of the Parson's tale is a lengthy analysis of each of the Seven Deadly Sins, with a discussion of the opposite of each sin: pride/humility; envy/love of one's neighbors; anger/meekness, mild behavior;sloth/industry, work;avarice/charity;gluttony/abstinence;lechery/chastity.
The theme of the Parson's tale, then, is that man is inherently sinful and cannot reach heaven without a fundamental understanding of his sins and then consciously repent. In order to gain that knowledge, man has to study the nature of the Seven Deadly Sins so his repentance can be genuine (because it is based on knowledge and understanding). The tale's moral is that man cannot reach the heavenly Jerusalem without repentance.
Many critics have commented on the "rightness" of ending the Canterbury tales with a serious discussion of the most important pilgrimage of all--man's journey to heaven.