a. Apostolicity and Apostolic Succession. One of the main criteria for the legitimacy of institutions and doctrines in the early Church was whether they were "Apostolic". That is, during the 2nd century and after, those trying to maintain teachings and practices that went back to Jesus himself defined the most authentic as those that had been approved by the Apostles themselves. For example, the books that were admitted to the NT were all believed to be Apostolic (that is, written by the Apostles). During the medieval period the most important institution to be undergirded by this doctrine was the hierarchy of the Church itself. The RC Church understood Peter as having been invested as the first Pope by Jesus himself (when Jesus punned on his name), and they regarded all subsequent Popes as the legitimate, divinely sanctioned successors of Peter. With the acceptance of this idea by almost everyone in medieval Europe, the Popes of the period had a very powerful doctrinal and political position, since they were regarded as Christ's specially selected representatives on Earth.
Apostolicity and Apostolic Succession were especially important for the nature of religious politics and culture during the medieval period, especially when these doctrines are understood in combination with other important Roman Catholic beliefs. For example, the belief that all Popes succeeded Peter in holding the divine mandate as the head of all Christians on Earth coincided with beliefs about the Office of the Keys, which granted the Pope the authority to exclude people from access to eternal salvation. Thus, the Pope's power to excommunicate unbelievers, heretics, and reprobates was not limited to earthly society, but extended even to the afterlife. Similarly, since the Church with the Pope at its head was understood to be the divinely inspired agent of God's will on earth, it was regarded as the sole agency who could render reliable interpretations of scripture in matters of Christian doctrine. That the Pope specifically and the Church generally were believed to bear the legitimacy of apostolic succession made their position practically unquestioned among the ranks of believers. Even Kings felt obliged to kneel in submission to the Pope, when he declared them in violation of church doctrine.
Luther's challenge to the doctrine of Apostolicity was a very gutsy and dangerous thing to do, and he would not have been able to pull it off, were it not for the political support he received from a local German nobleman. (In fact, Luther so disdained the Pope that he wrote many sarcastic and damning diatribes against the Holy See.) Still, Luther would never have opposed the Pope in this way, if he had not had doctrinal objections to the Roman Catholic doctrine. His opposition was formulated in terms of "the Priesthood of All Believers."
b. The Nature of Revelation and the Position of the Bible. The RC Church maintained that the Bible was its authoritative Scripture and the most important of God's revelation on Earth, but it also maintained that the Holy Spirit continued to work through the RC Church. Its authoritative documents were, therefore, also inspired and part of God's instruction to human beings about proper belief and behavior. Furthermore, the RC Church insisted that true understanding of the Scripture could only be achieved in conjunction with authoritative Church documents.
This issue had been extremely important for the history of biblical interpretation going back to times before the life of Jesus himself. Early on in the history of scriptural interpretation in 2nd Temple Judaism, there was widespread disagreement among people who revered traditions from ancient Israel about the meaning of ancient sacred texts. This was, for example, an issue that helped to define the disagreements between the Pharisees and the Sadducees in the late 2nd Temple period. More importantly for the history of Christianity, it was observed shortly after the life of Jesus by theologians in the early church that there were significant points of discontinuity between the teachings of Jesus and the theology of more ancient Hebraic traditions. One early theologian, Marcion, even suggested that the Hebrew Scriptures be abandoned, since their theologies did not reflect well on Jesus' message. Marcion's view was eventually branded as heretical, and the Old Testament was retained as a part of Christian scripture. But it was recognized from this time forward that a hermeneutical device was needed to account for discrepancies internal to the scriptures themselves, and others between the scriptures and established church doctrines. Towards this end, for example, a standard distinction was drawn between the "literal" and "spiritual" meaning of the Bible. By drawing this distinction, theologians were able make sense of passages that seemed to have little religious merit, or even seemed to contradict important Christian doctrines.
The authority of the Roman Catholic Priesthood, with the Pope at its head, to determine the legitimate doctrinal interpretations of scripture was extremely important for maintaining the coherence of the tradition. Nonetheless, Reformers like Luther attacked this doctrine with one of their own which asserted that the scripture (Bible) alone is sufficient in determining the truth of God's revelation (see Sola Scriptura.) The Protestant challenge to Roman Catholic doctrine on this point allowed over the next several centuries a flood gate of revisionist theology to shake to its very foundations Christian views about revelation, church doctrine, and the nature of God.
c. The Nature of Salvation -- Heaven, Hell, and Purgatory. Perhaps the single most important doctrinal concern of Christian history has been with how human belief and behavior on Earth influences one's fate in the afterlife. The official RC position on this came to be (at least by the year 1000) that there were two main destinations for believers and unbelievers: Heaven and Hell. Heaven was a place where a person's soul went at death to enjoy eternal bliss, if the person had believed in Christ, done good works in plenty, and participated in the sacred rituals administered by the RC Church. Hell was the place of eternal torment established for those who did not do what was necessary for entry into Heaven. A third abode in the afterlife, Purgatory, was a sort of "holding pen" for those who were otherwise fit for life in Heaven, except that they had committed too many sins on earth. Those people who went to Purgatory were "purged" of the effects of their misdeeds on earth by suffering there for a time (actually, a very long time), but eventually (when their sentence was fulfilled) they were allowed to enter Heaven. Dante's Divine Comedy gives the most famous contemporary poetic rendition of these realms as conceived in Medieval Europe.
The doctrinal battles that were fought between the Roman Catholic Church and the Protestant Reformers turn most heavily on the nature of and means to divine salvation from Hell in Heaven. To my knowledge, no one on either side of this divide seriously and publicly disputed the literal existence of either place, nor the importance of determining the proper dogmatic approach to attaining salvation. They all believed in Heaven and Hell, and they all formulated Christian doctrine with an eye to determine the best way to save souls from everlasting torment. Still, the Protestant attack on the existence of Purgatory opened the door for later disputes about the existence of Heaven and Hell themselves.
Protestants argued that Purgatory did not exist for two reasons. First, there is no mention of Purgatory in the Bible. Believing as they did in the doctrine of Sola Scriptura, Protestants concluded that Purgatory was a sophistical invention of the Church that could be used to milk money from the poor by playing on superstitious fears (see Dispute about Indulgences). Second, Protestants believed that good deeds played no part in divine salvation. Thus, there was no need for a place where evil deeds could be atoned for.
The challenge to the existence of Purgatory did not lessen the virulence of Protestant threats of hell to intimidate heretics and backsliders. Still, the challenge to Purgatory would be expanded eventually to threaten Christian beliefs about Heaven and Hell as well. As you will see, biblical evidence for the existence of Heaven and Hell, per se, is non existent. Biblical scholars focused on this, and later theologians came to realize the speciousness of even early Protestant doctrines about the afterlife.
d.Access to Salvation through the Church -- Office of the Keys, the Sacraments, Excommunication, and Indulgences. Since these three places were conceived as real (i.e., not merely poetic images), they had a powerful effect on the minds of the people of the time. The RC hierarchy, which claimed to control access to Heaven and to be able to commend people to Hell, had a great deal of power over the people who believed these things -- and this included practically everyone in Europe at the time.
When the Church found itself in need of funds to complete the building of St. Peter's Cathedral in Rome, it sent out salesmen to regional fairs and pilgrimage sites to sell indulgences. Indulgences were printed up by intinerant printers using the newly invented printing presses of the time. It is said, even, that the first advertising jingle of modern times comes from this period. Roughly translated it reads: "When a coin into the coinbox rings, a soul from Purgatory soon will spring." Such excesses constituted the spark that ignited the Reformation.