Pope Francis has denounced the resistance he is encountering in reforming the Vatican bureaucracy, saying some of it is inspired by the devil and that the prelates who work for him must undergo a process of "permanent purification" to serve him and the Catholic Church better.

For the third year in a row, Francis took the Vatican bureaucracy to task in his annual Christmas greeting. He said his reform process isn't aimed at a superficial face-lift, but rather a profound change in mentality among his collaborators.

He said Thursday: "Dear brothers, it's not the wrinkles in the church that you should fear, but the stains!"

In 2014, Francis denounced the 15 "spiritual ailments" Vatican bureaucrats suffered. Last year he listed the "catalogue of virtues" they should show.