In the end, did the Crusades weaken or strengthen the power the Church had in European society? Why?
In the short term, the Church enhanced its wealth and power through the Crusades. Conquered lands brought with them enormous wealth and property, most of it coming from looted churches and monasteries. The acquisition of land brought with it political power, and, at this time, the Church was a major player in international politics. So initially, the Crusades greatly expanded the Church's influence over vast tracts of territory from which it had previously been excluded. The spiritual authority of the Church had also been enhanced as it gained a lot of credit for taking the fight to what were regarded as heretics, the enemies of Christianity.
Long-term, however, the Crusades proved damaging to the Church. Atrocities committed by the crusaders against Byzantine Christians led to an irreparable breach between Western and Eastern Christianity, one that still persists to this day. The Crusades also led to the downfall of the feudal system in Europe, giving rise to the centralization of power and the enhanced authority of kings. As a result, the Church was now faced with a dangerous threat to its own temporal power. Relations between Church and secular rulers were a running sore for centuries, creating a permanent tension between the two sources of power that contributed to the division and eventual disintegration of Western Christendom.
There are at least two major ways in which the Crusades helped increase the power that the Church had. One of these ways was psychological and one was more tangible.
The psychological impact came from an increase in the prestige of the pope. The pope came to be seen as the person at the head of the Church, defending Christianity from the Muslims. In this way, the Church gained strength because people felt that it was leading all of Europe in this huge undertaking. Europeans coalesced behind the Church's leadership in ways similar to how people fall in line behind their leaders in times of war.
The more tangible impact of the Crusades was that it made the Church richer. Kings and nobles leading their men on Crusades typically needed loans. The Church was the major source of these loans. This made a great deal of money for the Church.
In these ways, the Church gained power from the Crusades both in terms of what people thought of it and in terms of how much wealth it had.