Until that time there were two centers of the Roman Church. One was in Italy, and the other was geographically in modern Turkey. In the 1400's the Ottomans took over Turkey, and that caused a weakening of the Roman Church.
The 1400's is also about the time of the Renaissance, and new ideas started coming into Europe from far the Near East for the first time since before the middle Ages. Anyway, when people were exposed to new ways, they questioned the old ways.
