(c) In doubtful cases the decision whether or not a particular kind of communication is lawful or unlawful pertains to the Ordinary (Canon 1258).
962. Participation of Catholics in non-Catholic services is either active or passive. (a) Participation is active when one takes a part or fulfills some function in an act that is an official expression of the worship and belief of a sect, even though this takes place outside a church, or is not open to the general public.
(b) Participation is passive, if one merely assists as a spectator, and not as a worshipper, at something pertaining to non-Catholic worship.
963. Sacred things in which communication is possible are of three classes:
(a) the chief acts of divine worship (i.e., Sacrifices, Sacraments, sacramentals);
(b) the secondary acts of divine worship (such as prayers, processions, vows, oaths, the Divine Office, hymn singing, scripture reading, etc.). In the Protestant denominations some one or other of these is, as a rule, the central or distinctive service, although some have other proper features of their own, such as the silent meeting of the Quakers, the seance of the Spiritualists, the march of the Salvation Army, the charity kiss of the Dunkards;
(c) places (e.g., churches, lodge rooms, cemeteries), times (e.g., days of feast or fast), and objects (e.g., images, badges, aprons, banners, robes), pertaining to divine worship.
964. It is unlawful for Catholics in any way to assist actively at or take part in the worship of non-Catholics (Canon 1258). Such assistance is intrinsically and gravely evil; for (a) if the worship is non-Catholic in its form (e.g., Mohammedan ablutions, the Jewish paschal meal, revivalistic “hitting the trail,” the right hand of fellowship, etc.), it expresses a belief in the false creed symbolized; (b) if the worship is Catholic in form, but is under the auspices of a non-Catholic body (e.g., Baptism as administered by a Protestant minister, or Mass as celebrated by a schismatical priest), it expresses either faith in a false religious body or rebellion against the true Church.
965. It is unlawful for Catholics to simulate active assistance in the worship of non-Catholics, for, while the non-Catholic rite would be avoided, something which appeared to be that rite would be done, and thus profession of faith in it would be given.
(a) Hence, it is not lawful to do an indifferent act which bystanders from the circumstances will have to conclude is an act of false worship. Thus, Eleazar would not eat lawful meat which was put before him in order that he might pretend to eat the meat of sacrifice after the manner of the heathen (II Mach., vi).