[UPDATE November 22, 2017]
The multiple historical and philosophical problems which are hidden in these affirmations cannot be discussed here. It is certainly not necessary to emphasize that following the Second Vatican Council the Catholic Church too is pressing in the direction of collaboration between all men of good will. Nevertheless, becoming a member of Freemasonry decidedly exceeds this legitimate collaboration and has a much more important and final significance than this.
Can. 1374 A person who joins an association which plots against the Church is to be punished with a just penalty; however, a person who promotes or directs an association of this kind is to be punished with an interdict.
– Penny Catechism Q. 204. Is it sinful to belong to a Secret Society?
It is sinful to belong to any Secret Society that plots against the Church or State, or to any Society that by reason of its secrecy is condemned by the Church; for St. Paul says: ‘Let every soul be subject to the higher powers; he that resisteth the power resisteth the ordinance of God; and they that resist purchase to themselves damnation’ [Rom 13:1-2]