31 We pray and beseech you, venerable brethren, to join your efforts with Ours, and earnestly to strive for the extirpation of this foul plague, which is creeping through the veins of the body politic. You have to defend the glory of God and the salvation of your neighbour; and with the object of your strife before you, neither courage nor strength will be wanting. It will be for your prudence to judge by what means you can best overcome the difficulties and obstacles you meet with. But, as it befits the authority of Our office that We Ourselves should point out some suitable way of proceeding, We wish it to be your rule first of all to tear away the mask from Freemasonry, and to let it be seen as it really is; and by sermons and pastoral letters to instruct the people as to the artifices used by societies of this kind in seducing men and enticing them into their ranks, and as to the depravity of their opinions and the wickedness of their acts. As Our predecessors have many times repeated, let no man think that he may for any reason whatsoever join the masonic sect, if he values his Catholic name and his eternal salvation as he ought to value them. Let no one be deceived by a pretense of honesty. It may seem to some that Freemasons demand nothing that is openly contrary to religion and morality; but, as the whole principle and object of the sect lies in what is vicious and criminal, to join with these men or in any way to help them cannot be lawful.
Source: Humanum Genus (Vatican.va)