Papal Ban of Freemasonry

The leadership of the Roman Catholic Church has long been an outspoken critic of Freemasonry, first prohibiting Catholics from joining the fraternity in 1738.

Since then, the Vatican has made several pronouncements forbidding Catholics from becoming Freemasons under threat of excommunication.

Although there was some confusion of the issue following Vatican Council II, the Church still prohibits membership in the Freemasons because it believes that the principles of Freemasonry and the teaching of the Catholic Church are irreconcilable.

A 1983 statement from the Sacred Congregation for the Doctrine of the Faith approved by Pope John Paul II stated "The faithful who enroll in Masonic associations are in a state of grave sin and may not receive Holy Communion." and "... membership in them (Masonic associations) remains forbidden."

http://en.wikipedia.org/wiki/Papal_ban_of_Freemasonry