The following is the condensed text of a reply I gave to a recent convert to the Catholic faith who wanted to know if Catholics could belong to Freemason groups, especially because they do so much charity work for the community:
Canon Law number 1374, which is clarified by a Nov. 26, 1983, document signed by the man who is now pope, specifies that Catholicism is incompatible with membership in a masonic organization.
There are a few reasons for this. The first is that Masons have historically plotted against the Catholic Church and have aimed at its demise. While it may appear that today's American Mason groups do not have this goal, membership in a Masonic lodge requires a pledge of worldwide solidarity, and a Catholic cannot hold membership in an organization which aligns itself with enemies of the church that Christ founded.
Another reason is that Masonic oaths require a member to protect all the secrets of the Mason groups. Unfortunately, many of these secrets are not revealed until long after the oath has taken place, so by the time someone realizes they are contrary to his belief system, he has already bound himself by oath to protect and adhere by them.
Finally, the roots of masonry are pagan in origin, and most of its rites are built upon these roots. In fact, while as Catholics we believe in the divinity of Christ and the Trinity, Masonry requires belief in a generic "grand architect".
The Freemason meetings that are open to the general public are rather benign. Like with most fraternal organizations, much of what the masons believe is revealed only at higher levels of membership, only after one has taken oaths to protect that secrecy. In addition, higher levels of membership also requires participation in meetings which will be a bit more revealing than what they offer for the general public. Remember, as with any organization, the meetings that are offered to non-members are intended to be a hook into the organization, anything a person witnesses there will appear "completely harmless" so that prospective members are not turned off by the organization.
It is admirable that some want to get involved in charity work. Fortunately, a person does not need to be involved with the masons to do so. There are many civic organizations, like the Lions Club or the Elks Club, which do great charity work. In addition, the Knights of Columbus is a Catholic organization which does tremendous charity work.
The religious beliefs and practices that one must subscribe to in order to be a mason are reserved for committed, pledged members. Wanting to do charity work is simply not a good reason to compromise the integrity of one's faith by an organization which is contrary in beliefs and practices.
The community work that the Masons do is admirable, but this does not excuse adherence to their beliefs. Consider this analogy, the KKK also is known for community work - readers probably remember the controversy over their adopt-a-highway efforts. However, nobody would ever suggest that their service to the community is a good reason to join that "fraternal organization", considering how offensive their beliefs are.
Considering this, it is easy to realize the Church's problem. Even if many American lodges are fairly benign, if Freemason groups have at various points demonstrated anti-Catholic agendas, can we really expect the Church to explore each and every one, especially when faced with the problems of vows of secrecy? Is it not the more responsible position for the Church to ban membership outright, especially since, as you will acknowledge, one does not realize the deepest parts of commitment to a mason group until you have reached the upper levels of membership. Isn't this what a good parent would do?
For evidence of an anti-Catholic agenda from relatively recent times, the witness of the events in Portugal and Mexico when Freemasonry attacked the life of the Church in those countries should be sufficient for any Catholic to at least acknowledge that Freemasonry historically hasn't had a particularly benevolent attitude - to say the least - towards Catholicism.
Also worthwhile reading are the three encyclicals of Pope Pius XI responding to the Freemason attack on the Church in Mexico during his pontificate and continuing up to our present day:
The anti-Catholic roots of Freemasonry were in evidence in the violent attempts to overthrow the Church in Mexico, which led to many martyrdoms and expulsions from the country.