What weakened the catholic church and allowed the reformation to begin?

Several things had weakened the Roman Catholic Church in the century or so prior to the Reformation.  When the Black Death struck Europe in the 14th century, the church did little to comfort people during the crisis -- and those priests who did help parishioners during the plague became victims of the plague themselves.  So the integrity of the clergy and confidence in the church were impacted. There was much corruption and politicking in the church in the days prior to the Reformation, which the reformers spoke out against mightily.

Also there had been a rise in the power of kings and princes over their own territories, standing up to the power of the pope and the church.  So when the Reformation did break out, there was protection for Martin Luther from the prince in his territory.  The pope was not able to extract him from Saxony and have him brought to Rome.