Would it be considered a sin to sometimes assist at and receive Communion at Masses celebrated by a priest from the Society of St. Pius X (SSPX)?
The Society of St. Pius X is headed by bishops who were consecrated by the late Archbishop Marcel Lefebvre in defiance of the wishes of Pope John Paul II. In response, the Archbishop and the bishops he illicitly consecrated received automatic excommunication from the Church. John Paul II made this appeal to Catholics in his apostolic letter Ecclesia Dei:
In the present circumstances I wish especially to make an appeal both solemn and heartfelt, paternal and fraternal, to all those who until now have been linked in various ways to the movement of Archbishop Lefebvre, that they may fulfill the grave duty of remaining united to the Vicar of Christ in the unity of the Catholic Church, and of ceasing their support in any way for that movement. Everyone should be aware that formal adherence to the schism is a grave offence against God and carries the penalty of excommunication decreed by the Church’s law (ED 5c).
In light of this, it does not appear to permissible for a Catholic to go to SSPX Masses and especially not to receive Communion. Although the Communion is validly consecrated, it is consecrated under illicit circumstances and to receive it from the SSPX would imply a unity with the SSPX that does not exist at present. Far better would be to pray for the progress of the Holy See’s negotiations with the SSPX that the SSPX may be brought back into full communion with the Church.