What were the factors, internal and external, which affected the Church, and resulted in a decline of prestige and a loss of power by the beginning of the fifteenth century?
Post number 9 is correct, but surely the question refers specifically to Europe, and even more specifically to the Catholic Church. All of the factors listed above are important, but I'd also think about how the Church was perceived on a grass-roots level. It was there that all this corruption and ignorance by priests was most intense, and where it had the most important epistemological implications. The printing press was important, but it's also possible, as the almost yearly suppressions of heresies by the Holy Office demonstrate, that common people throughout Europe were already becoming fed up with the abuses of the Church. In other words, the Protestant Reformation took root in soil that was already pretty rich.
This is an interesting thread. It is possible to argue against all of these points. It is possible to argue that the church has grown substantially from the 1500's. Perhaps there has been a decline in France and Europe, but how about Asia, Africa, and South America? The church in these areas has grown. For example, it is estimated that there are 100,000,000 Christians in China. That is an amazing number. In the last twenty years, there has been explosive growth in Brazil. There are more than 50,000,000 Protestants there now. Even in America, immigrant churches are growing. From these figures, you can argue that the church has merely shifted.
One should not overlook the rampant corruption and hypocrisy which characterized the Popes of this era; a time when honest and upright pontiffs were the exception rather than the rule. The office of Pope was more of a political prize than a respected position of church leadership. Julius II, who commissioned the painting of the ceiling of the Sistine Chapel was known as "Papa terrible," because he would rather fight in a war than conduct church business. He was commonly known as the warrior pope. Rodrigo Borgia, the Spaniard who became Alexander VI raised two of his own children in the Vatican, Lucretia and Cesare. In a particularly heinous instance, Alexander had a group of condemned men delivered to the Vatican courtyard. The men expected a pardon. Instead, Alexander's son Cesare opened fire on them with a rifle killing them, after which Alexander complemented them on their marksmanship. Leo X, of the deMedici family, once commented, "God has given us the papacy, let us enjoy it." He not only enjoyed the Papacy, but also lusting after teenage boys. The worst example, some years earlier, was the youngest pope, John XII, who was elected at age seventeen but died when he was twenty five while in the act of adultery with another man's wife. These actions along with others noted above caused the Papacy to lose tremendous prestige and with it it's influence.
#3 makes a very good point. We need to remember that up until the creation of the printing press the church held a monopoly of knowledge and control. Suddenly you had people reading the Bible for themselves, and therefore free to question and challenge the accepted views of the church. This represented a massive change in the way that things had been and in particular started the erosion of the control and power that the church possessed.
There were so many new ideas at that time battling for supremacy. Inventions like the telescope and printing press, as well as feuding, would start to chip away at the church's power.
The rise of royal power and the growth of nation-states as early as the 14th century began to fracture the foundation of the Roman Catholic Church. Kings, emperors, and even popes were undermining church authority in pursuit of their own agendas. The internal corruptions of priestly greed and abuse of power led those faithful who had survived the Black Death to question the sincerity of their church leaders. By the 16th century church reform was a very hot topic. Reformers, Martin Luther, John Calvin, John Knox and many others were responsible for spreading their ideas throughout Catholic Europe. In addition, as noted in post #3, the printing press was key to the Protestant Reformation. As literacy increased more people read and began questioning the church's authority, as well as church doctrine.
I'll second post #3: the printing press was the first real push for education beyond what was given by trades and church. With books available, and with some effort to learn how to read, people could educate themselves without relying on a biased or agenda-driven curriculum. It was the spread of knowledge that gave people the ability to move away from dogma and towards their own prosperity and well-being, and ultimately the freedom to decide for themselves what they wanted from life.
The invention of the printing press, which revolutionized so much of life. While there weren't huge numbers of people who could read at the beginning of the fifteenth century, there were some - and more as books began to become somewhat more readily available. Because one of the first books to become available was the Bible, people began to read it for themselves and began to question some of the interpretations and practices that had been dictated by the Church in a less informed period of time.
I'd say the biggest was the internal fighting within the Church. Throughout the 1300s, the papacy, for example, had been a source of contention. There had been multiple men claiming to be the true pope. This did not help the Church in terms of prestige.