Upvote:4
It is forbidden for Catholics to engage in communicatio in sacrisβjoining non-Catholics in their worship services:
1917 Canon 1258 Β§1 It is not licit for the faithful by any manner to assist actively or to have a part in the sacred [rites] of non-Catholics.
cf. Dom Augustine's A Commentary on the New Code of Canon Law on '17 Can. 1258
This law is expressed in the 1983 code, in the section "Delicts against religion and the unity of the Church," as:
1983 Can. 1365 A person guilty of prohibited participation in sacred rites (communicatio in sacris) is to be punished with a just penalty.