The Renaissance was marked by secularization, and the papacy was no exception. The Popes were becoming so concerned with art, political power, and other worldly matters that they neglected their spiritual duties, to varying extents. This neglect caused the Church to degrade, and invited the Protestant Reformation.
Two events, the Avignon Papacy and the Great Western Schism, greatly effected the condition, unity, image, and privileges of the Church. The Avignon Papacy was a period from 1305 to 1377 in which the entire papacy moved from Rome to Avignon in France. This period was marked with corruption. People filled church offices simply to receive their income, Cardinals attempted to gain extra power, along with other abuses. The Great Western Schism was a split in the Church that started in 1378 (just a year after the papacy had been moved back to Rome by Pope Gregory XI), where two opposing Popes operated independently of each other at the same time. One was based in Rome, and the other moved back to Avignon. The schism was healed in 1415, when at the Council of Constance both the Roman and Avignon Popes were declared illegitimate, and a new Pope, Martin V, was elected. During these terrible events in Church history, various political units had seized this opportunity to take privileges belonging to the papacy. It was thought by the Popes that the best way to respond would be to do what a temporal political leader would do, that is, engage in diplomacy, forge alliances, and even wage war.
Nicholas V, who was Pope from 1447 to 1455, was afraid that the disorder of Italy at the time would invite the intervention of outside forces, namely the French. He formed alliances between Venice, Milan, and Florence, and viewed the papacy as instrumental in Italian politics. Several Renaissance Popes engaged in nepotism, chief among them were Calixtus III (r.1455-1458) and Sixtus IV (r.1471-1484). Calixtus appointed as Cardinal his nephew, Roderic Borgia, who would later wind up as Pope Alexander VI (r,1492-1503), one of the most notorious Popes in church history. He had several natural children, and allegedly even committed murder (although this is unlikely). Julius II (r.1503-1513) waged war against Venice, which he thought was becoming far too powerful in the north of Italy. He formed a coalition with the French to attack Venice, and attacked cities that refused to join in person. He won the war with Venice in 1509.
Concerning the arts, Nicholas V again comes to mind. He saw Rome as the center for the arts in the world, and summoned many artists to Rome. He had people translate great works of classical literature, keeping with the Renaissance tradition, and established what would become the Vatican library. He focused himself so much on art that he neglected necessary reforms. Julius II was also an art lover, and formed a collection of sculptures, including Laocoön, that would develop into the Vatican museum. Leo X (r.1513-1521) was so preoccupied with the arts that he neglected the enforcement of necessary reforms, and was oblivious to the Reformation, which was gaining steam around him.
As in many other fields, the papacy was marked with secularization during the Renaissance. Popes concerned themselves with politics and the arts, while ignoring the fact that reform was necessary. The Protestant Reformation was not dealt with properly during the Renaissance papacies, and it gained speed due to the Renaissance Pope’s negligence.