In his annual Christmas greeting, the pontiff denounced the "hidden" resistance he is encountering in reforming the Vatican bureaucracy.
Since 2013, he has been trying to introduce changes within the administration to modernise and make people more accountable.
He's previously spoken of the need to bring about a profound change in mentality rather than a superficial face-lift.
In this latest address, he said: "Dear brothers, it's not the wrinkles in the church that you should fear, but the stains."
He's given the priests, bishops and cardinals who work for him twelve guidelines which are inspiring his reform process but said he understands why there would be some resistance. However, he added that there is good resistance and bad.
Positive resistance is an open willingness for dialogue, but then there is "malevolent resistance," he said "when the devil inspires nasty intentions."