Why do the religious seek purity?
“Because their scriptures command them to,” one might be tempted to respond. Jesus tells his disciples, “Do not store up for yourselves treasures on earth… But store up for yourselves treasures in heaven,” (Matthew 6:19-20) ; the Quran instructs, “[F]ollow not [personal] inclination, lest you not be just,” (4. An-Nisa’ 136) ; the Bhagavad-Gita commands, “All… activities should be performed without attachment or any expectation of result.” (18.6) 
This answer is unsatisfying. Why do all of these religions command us to purity in the first place? Supposing that religious faiths arise and evolve spontaneously in response to people’s spiritual needs, it would follow from this that purity’s ubiquity in religions implicates it as a fundamental component of the human religious impulse. Were we to suppose, instead, that God created the world’s religious creeds (or, Heaven forbid, just one), we would still have to understand purity’s role in our earthly lives, and whether — provided that this role were a positive one — God’s presiding approval were prudence or coincidence.
I’ll explore purity as a human phenomenon, then — one inextricably linked with religion, and one in need of explanation.
Guilt is one factor which can simultaneously explain these distinct tendencies towards purity and religion. On the one hand, guilt incites purity: the guilty must expiate their sins. Those overcome by guilt might desire to experience suffering — to undergo penance — to a degree proportional to their perceived wrongdoing, and this penance often takes the form of purity or asceticism. On the other hand, guilt induces religion: the guilty have particular reason to seek mechanisms for the construction of conceptions of the ideal. Using religion to develop visions of the perfect — of the guiltless — the guilty may attain direction. Guilt, then, engenders two complementary needs — the excision of past wrongs and the formulation of a future right — and these needs can be simultaneously met by, respectively, purity and religion.*
Anxiety, more broadly, can also explain purity and religion’s coexistence. Anxiety instills in one a conviction that all is not right, and that measures must be taken to remedy things. The imposition of ascetic strictures can resolve this tension. (The anxiety may be either mindlessly short-circuited — consider classic, and useless, penances such as the bodily mortifications — or productively remedied — many channel anxiety into the constructive pursuit of their goals.) Simultaneously, anxiety, like guilt, generates a desire to construct a destination — a pathway out. These glimmering projects typically involve realization of desires and aspirations. Religion facilitates deliberation upon perfection, and it can aid in the construction of these visions. Anxiety, like guilt, creates simultaneous needs — the casting aside of imperfection and the understanding of perfection — which can be respectively pursued through purity and religion.
Most generally of all, purity and religion serve those who keenly perceive a discrepancy between their own imperfections and their ideas of perfection. In these “purists”, purity and religion serve opposite and complementary goals: the eradication of perceived faults and the cultivation of a vision of the perfect. The phenomena are closely intertwined.
And they work. Purity and religion, the functional duo, often furnish the discipline and hope necessary to push through difficult periods and toward the attainment of dreams. They’ve made good, modest, prudent, and successful people out of many.
Nietzsche, asceticism’s most strident critic, was right, then, in a way, when he declared asceticism the “chief weapon in the battle against long-drawn-out pain and boredom… an excuse to hibernate at last.”  Asceticism does appeal to the struggling. He was wrong, though, in that this particular hibernation nourishes and inspires, and gives way, eventually, to a better life. “[The] human will… needs an aim,” Nietzsche writes, “and it prefers to will nothingness rather than not will.” Most of all, though, it prefers to will its own triumph.
*Guilt might also lead to religion through a different pathway: Many religions feature mechanisms for the absolution of guilt, which could make these religions appeal particularly to the guilty. I’ve shied away from using this reasoning because it’s dangerously circular: That so many religions include mechanisms for the absolution of guilt could reflect merely the very prevalence of the guilty among their ranks which we originally sought to explain, and we must explain why the guilty flocked to these religions in the first place. Once a primary pathway has been established, then this circular secondary pathway becomes valid.
- The Bible, Gospel of Matthew, Book 6
- The Quran, Book 4
- The Bhagavad-Gita As It Is
- Nietzsche’s On the Genealogy of Morality