The Church’s declaration on Freemasonry exposes Catholic Masons to a number of penalties under canon law. For example, a Catholic who is aware that the Church authoritatively judges membership in Freemasonry to be gravely sinful must not approach Holy Communion (c. 916). The Church imposes the duty upon all grave sinners not to make a sacrilegious communion. Such a Catholic Mason who is aware of the grave sin must receive absolution in a sacramental confession before being able to receive communion again, unless there is a grave reason and no opportunity to confess (c. 916). This confession, in order to be valid, also requires the Catholic Mason to renounce his Masonic membership.
Further, because membership in Freemasonry is an external or public condition, the Catholic Mason can be refused Holy Communion by the pastors of the Church for obstinately persevering in his Masonic membership (c. 915). Such a Catholic Mason would also be forbidden from receiving the Anointing of the Sick (c. 1007) as well as ecclesiastical funeral rites if public scandal were to result (c. 1184, §1, °3).
Canon 1364 also imposes an automatic excommunication upon apostates, heretics, or schismatics. This canon could also apply to Catholic Masons. If, for example, a Catholic Mason embraced the theological teachings of Freemasonry that the Church has condemned (indifferentism, syncretism), he would be in heresy by virtue of his belief in these teachings. Further, if a Catholic Mason knew the Church opposes membership in Freemasonry, and yet adamantly and persistently refused to submit to the pope’s authority in precluding his membership in the Lodge, he may also find himself in schism. Catholic Masons could also be subject to canon 1374 which imposes an interdict or just penalty upon those who join associations that plot against the Church.
For the canonical penalties to apply, the Catholic Mason would have to act in a gravely imputable way (that is, the Catholic would have to be aware of the Church’s teaching on Freemasonry and, after being warned about it, choose to disregard it). In my personal experience, a fair number of Catholic Masons do act in a gravely imputable way in regard to their Masonic membership. In these cases, the canonical penalties, including excommunication, apply. The Church's penalties are not meant to alienate the person on whom the penalty is levied. Instead, the penalties are meant to communicate to the person the gravity of his conduct, encourage his repentance and reconciliation with the Church, and bring him back into the one fold of Christ. After all, the mission of the Church is the salvation of souls.
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Charles Webster Leadbeater - The Hidden Life In Freemasonry