To help you understand the background of Wolfram's poem I have written the following notes on the Manichaean religion and its medieval descendants.
Manichaeanism was founded in about 240 CE by Mani (c.216 - c.276 CE), who considered himself an apostle of Christ. The religion combined elements of Christianity, Gnosticism, Zoroastrianism, Buddhism, and Taoism; he described it as a mighty current combining the streams of the other religions. (St. Augustine was Manichaean before converting to Christianity.) Manichaeanism spread quickly until it had many followers in the Middle East, throughout the Roman Empire, and as far east as China. In the West, Manichaeanism disappeared after the sixth century, but Manichaeans were active in China until the thirteenth century. However, many Manichaeans avoided persecution by disguising themselves as Christians, Buddhists, Taoists, etc.
Several medieval religious movements were descended from Manichaeanism. Their adherents were called Cathars because of their concern with purity (Greek, catharos). One of the most popular movements was the Albigensian, which arose in southern France (Provence) in the twelfth century. (This is the area from which Wolfram claimed that he got the story of Parzival - from Kyot of Provence; recall that Wolfram completed Parzival about 1212.)
The Albigensians had powerful political protectors and won many converts because their asceticism compared favorably with the excesses of the contemporary Catholic clergy. Therefore Pope Innocent III sent poor preaching friars into the region to stop the spread of Albigensianism, including a group led by St. Dominic in 1205. Then, in 1208, after the murder of a papal legate, Innocent III launched the Albigensian Crusade, which lasted until 1229 and decimated Provence. Four years later, Pope Gregory IX established the Inquisition in Albigensian areas to root out the heresy. The Albigensians were quickly destroyed, along with most Provençal culture. (Other Catharist movements were exterminated by the fifteenth century.)
The Troubadours, the aristocratic poet-musicians of Provence, were often attached to the courts of nobles who were Albigensian or had Albigensian sympathies. Therefore many of them fled northward during the Albigensian Crusades, spreading their stories and art into other regions. The German imitators of the Troubadours were the Minnesingers ("love singers"), including Wolfram von Eschenbach.
I will outline briefly some Manichaean beliefs to illustrate the similarities with Wolfram's ideas about the Grail Kingdom. Mani claimed that his religion was a revelation from the Paraclete (the Holy Spirit), but his followers said that either Mani himself or his "Light Twin" was the Holy Spirit. (Therefore, we should recall how the Grail Stone was "recharged" each Good Friday when the Dove of the Holy Spirit brought the Host to it.)
Manichaeanism was a very dualistic religion, emphasizing the conflict of God vs. Satan, good vs. evil, life vs. death, light vs. dark, etc. According to Manichaean beliefs, the current era is characterized by a mixture of good and evil, with the light (divine power) being trapped by dark forces of matter. The goal of all Manichaeans was to free the light from the bondage of matter so that eventually all the divine substance would be reunited (as it had been at the beginning of time). The totality of entrapped light was called the Living Self (also, the Cross of Light, the Vulnerable Jesus, the Five Elements).
The Manichaeans were divided into two "orders" or "degrees": the higher were called the Elect (also the Chosen, Select, Religion Bearers, Perfect, Righteous, Holy) and were ascetic priests. The lower were called the Auditors ("Hearers"; also called the Faithful, Believers, and Catechumens - "Instructed"); they were responsible for guarding, supporting and caring for the Elect. Both men and women could belong to either order. We may compare the Elect to the Grail Lord, Grail Bearer, and other residents of the Grail Castle, and the Auditors to the Grail Knights, who protect and tend to the Grail Kingdom.
By rigorous discipline the Elect attempted to refine their bodies into instruments for freeing the imprisoned Living Self. This act of redemption was accomplished during a daily ritual meal, in which the light was released from particular foods, which were believed to have large concentrations of the most refined light. After the meal, the Elect sang hymns, which allowed the light to ascend to heaven. Thus, the task of the Elect was to work for the salvation of the world (the redemption of the Living Self), in which they were assisted by the Auditors.
The behavior of the Elect was bound by the Three Seals: Mouth, Hands, and Heart. By the Seal of the Mouth they were prohibited from eating meat or drinking wine. Once a day they ate food with the highest concentrations of light. By the Seal of the Hands, also called the Rest (of the Hands), they were prohibited from causing injury or pain to the light entrapped in water, fire, trees and living things. From a practical standpoint, this meant that they were unable to do any physical work, so they depended on the Auditors to bring them food (more on this below). By the Seal of the Breast they were prohibited from marriage, sex and lust. (The male and female Elect had no contact with each other.) Some Manichaeans said the Three Seals correspond to the Father, Son, and Holy Ghost. An alternative formulation divided the ethics of the Elect into Five Commandments: truth, non-injury, chastity, purity of mouth, and poverty. (The Elect were supposed to possess nothing beyond food for one day and clothing for one year.)
One was made one of the Elect by an elaborate ceremony (the Consolamentum) involving a laying-on of hands and combining the effects of baptism, confirmation and ordination.
The Manichaean church was headed by a sort of pope (in a direct line of succession from Mani), under whom there were 12 teachers, 72 regional directors, and many lesser officers. This elaborate organization helped the religion to spread across the Old World.
The Auditors were (necessarily) held to a lower standard of purity than the Elect. For example, they were allowed to hold jobs and to gather food; they were prohibited from killing, but could eat meat (although it was discouraged). Also, they were allowed to marry (monogamously) and have sex, although childbearing was discouraged, since it would cause more souls to be entrapped in matter (the rhythm method was advocated, according to Augustine). They were encouraged to treat their spouses "like strangers in their houses" (i.e. with courtesy, but avoiding entangling intimacy).
Sometimes the Auditors' ethics was codified in Ten Commandments. The list differed somewhat from place to place, and may have been adapted to local notions of morality, but a typical list of "thou shalt nots" included worshipping idols, lying, avarice, killing, adultery, stealing, teaching defects, magic, having two opinions about the faith, and neglect or lassitude in action.
Furthermore, the Auditors had a Twofold Discipline parallel to that of the Elect. The first task was triple: to fast one day out of seven, to pray to the sun and moon, and to make alms-offerings to the Elect. The second task was to give to Righteousness (that is, to be one of the Elect) either a child, kinsman, member of the household, slave, or a person redeemed from affliction. (Anyone willing to submit to the discipline could become a Manichaean.)
Because of their purity, the Elect would be rewarded after death by becoming "angels." To the extent that Auditors approximated the purity of the Elect, they too would be rewarded. The best class of Auditors - the Perfect - would be made citizens of the City of Light (governed by the deified Elect) and would be exempted from reincarnation. Less perfect Auditors would be reincarnated, but in a better state, in the best cases, as members of the Elect. Public confession and absolution of sins were used to encourage morality. It was especially important that the community be confident in the purity of the Elect, upon whom everyone's salvation depended. (Thus we may understand the danger to the community of an imperfect Grail King.)
In summary, the Elect and the Auditors existed in a symbiotic relationship. The Elect could not exist in their purity without the aid of the Auditors, but the Auditors could not attain salvation without the aid of the Elect. The Elect share from their heavenly wealth, the Auditors from their earthly wealth.