The relevant Vatican documents from the CDF in 2001 and 2008 - the 2001 document became a news event because it underscored how the traditional Nicene churches are different from the LDS Church, which does not embrace Nicene orthodoxy:

https://www.vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_20010605_battesimo_mormoni-ladaria_en.html

https://www.vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_20080201_validity-baptism-miralles_en.html

Example of practical application by diocesan legislation (though it contains equivocal matter, too: for but one example, Seventh Day Adventist baptism is listed in both valid and doubtful lists - leading me to suspect this was compiled by committee and never fully reconciled as a whole - other diocese have published lists as well): https://lacatholics.org/wp-content/uploads/2021/07/Valid-Invalid-Baptisms-.pdf

I cannot resist throwing a wrench into the works here, but how do we feel about Catholic organists playing for Protestant services because the pay is better than they can get at the local parish? Therefore many parishes limp along with the local piano teacher playing the organ as best she can, while the Protestants are accompanied by a well trained organist who is a Catholic.

I will now go hide in a bunker!!

Depends on where you are, Carol. In my area of the south, there are 300+ Protestant churches and a small handful of Catholic churches. There aren't enough Catholic parishes to employ more than 5 or so organists. Keep in mind some of the Catholic churches don't even have organs. If you want to play for a church, it will most likely be a Protestant one.

If the Church saw a need for a rule, it would have made one, and made it clear to all.

Yes indeed. The Church should make the rules clear, so they can be more clearly ignored.

bhcordova, those Protestant churches also tend to invest in better organs.

ServiamScores 11:45AM
Posts: 2,393
.... that this is absolutely not permissible. .... such worship is in fact an affront to our Lord since it is an ape of the true Faith, and to participate is to imply a communion that doesn't actually exist, and that furthermore, it could be injurious to one's faith to do so. Consequently, it is to be avoided at all costs.

Interesting that you bring up this issue.

I'm old enough to recall Catholics not being able to go to services of other religions for those reasons.

Then again, I was confirmed in 1967.

There is conflict between tradition and current law on these points. Directory for the Application of Principles and Norms on Ecumenism ordered by Pope John Paul II, 1993:

118. In liturgical celebrations taking place in other Churches and ecclesial Communities, Catholics are encouraged to take part in the psalms, responses, hymns and common actions of the Church in which they are guests. If invited by their hosts, they may read a lesson or preach.

133. The reading of Scripture during a Eucharistic celebration in the Catholic Church is to be done by members of that Church. On exceptional occasions and for a just cause, the Bishop of the diocese may permit a member of another Church or ecclesial Community to take on the task of reader.

Fr. Heribert Jone, Moral Theology, 1962 American edition:

125. Active participation in non-Catholic services is entirely forbidden (C. 1258 ). The natural law forbids participation in services that are heretical. If the service is one that heretics have in common with us, even though no scandal comes from such participation, it is at least forbidden by Church law.... It is forbidden to sing, play the organ or other instruments in the religious services of non-Catholics. But it is not forbidden to pray or sing privately with heretics if the prayers or songs are not heretical and no scandal is given. Whoever acts contrary to the prescriptions of C. 1258 and takes part in non-Catholic services is suspected of heresy (C. 2316).

126. Passive attendance at non-Catholic services is allowed for a good reason.... Passive assistance implies that no part is taken in praying, singing, etc.

127. Active participation of non-Catholics in Catholic services is forbidden in as far as it gives the impression that there is no essential difference between Catholic and non-Catholic faith or promotes indifferentism.... [Non-catholics] may not carry candles at liturgical functions, alternate at choir-prayers or participate in liturgical singing. For special reasons the Holy Office has allowed schismatic girls to sing with Catholic girls at liturgical functions. If no Catholic organist is available, a non-Catholic may play the organ for a time, scandal, however, being avoided.

Jone, as I understand, represents a middle-of-the-road preconciliar position, not by any means the most rigid stance possible. Even today in Germany, where the churches aren't exactly famous for their conservatism, it is rare for a Catholic church to have a Lutheran organist or vice versa, and there was even some controversy over a Catholic being hired as Thomaskantor just a couple years ago. In the United States it's quite common for musicians to work outside their own denomination; we have so many denominations and sects here that it can be difficult for churches to find someone of their own confession. My current parish went many months without an organist before I was hired, and my last two predecessors weren't Catholic.

Last year I visited an Orthodox cathedral (as a tourist, not a worshiper) and was somewhat surprised to see a sign in the narthex stating very clearly that non-Orthodox were not to participate in the services in any way whatsoever. I have never seen anything like that in a Catholic church, but I did see a sign posted at the entrance of a monastery church saying that Communion would only be given on the tongue, no Communion in the hand. These kinds of things seem less offensive in places frequented by tourists than in the typical parish church. I've drifted a little bit from the question, but we have to decide whether we're going to follow tradition or the current rules, and what exceptions we'll tolerate out of necessity. I know others would disagree, by I've personally been told by both FSSP and SSPX priests that it's lawful to play the organ for a non-Catholic church for pay. At a conference, an FSSP priest said he thought an argument could be made in favor of non-Catholic singers supplementing at least one Catholic singer, which sounds quite reasonable. Perhaps inquire with your diocesan office of worship as to whether your bishop has an policy on the regular use of non-Catholic cantors.

In a PiusV descended missa cantata the cantor is adding an ornamental rendering of what has been said silently by the priest, which is the 'operative' liturgy.

Sorry, @a_f_hawkins, but I reject this interpretation as unhistorical and false. It is the priest's sotto voce reading of the sacred text that is the duplication of the actual liturgical proclamation of it, not the other way around. Surely you wouldn't say that, when the pre-55 rubrics are followed, the celebrant's reading of the epistle and gospel constitute the "operative liturgy" (whatever that means) and that the chanting by the subdeacon and deacon is merely a ceremonial addition! I realize that the toleration of the Deutsches Hochamt, where the actual texts of the choral Proper and Ordinary of the Mass aren't even heard by the congregation, is an argument against my position, but surely that is the exception, not the rule. The mostly failed overemphasis on active participation seems to have led to a reactionary misunderstanding in some quarters of the Mass as something that the priest alone does (in persona Christi, of course), and both extremes have contributed in their own way to the prevailing musical mediocrity in so many churches by detracting from the dignity of what we do and viewing choral music as either an obstacle to congregational participation or more or less sophisticated liturgical background music.

francis - judging the sacramental reality is beyond my pay grade, but I hold that the bureaucrats of SRC regarded the priest's utterances as the only essential. That is why Fortescue was so contemptuous of them, and considered that "whatever beauty interest or historic value, or dignity, the Roman rite ever had has been utterly destroyed by the uneducated little cads who run that filthy congregation at Rome;" Unfortunately their attitude was widely disseminated among the clergy, which led many of them to lose their bearings when liturgical reform came in.

BHCordova,

Could you define "invalid"?

For Holy Week, other rubrics are permitted, by special permission already granted.

Bombarde16,
I will keep you and your good lady in my Rosary for the completion of your studies and the joy of Baptism. of course, I'm not the only one here who will be doing that.

Ask the two voices about transubstantiation and you will clearly hear the difference… unfortunately the Catholic may also be Protestant…

Yet, I guess that a large majority among faithful Catholics cannot tell the difference between Transsubstantation and Real Presence.

In turn-about-fair-play, we may reflect that numerous non-Catholic but Christian churches would react in horror at the thought of a Catholic reading scriptures in their churches. I myself have been on the receiving end of such myopia, evidence of how all of us place grave respect on the purity of our own denominational ethos - as well we should.

It seems the answer the question is, depends on your parish and your pastor. A case could be made for either prohibition of non-Catholic cantors or for inclusion of same. Therefore, I would say this is a clear case of subsidiarity and discernment on a case-by-case basis.

Agreed. I'm not entirely sure the ecumenical norms are even relevant in some cases. A non-Catholic hired by a parish to sing when no qualified Catholic is available seems like a very different situation from someone invited to sing a psalm as an official representative of a non-Catholic church or community for some exceptional occasion, permitted by the bishop.

Jeffrey,

Some things shouldn't need to be said.

Jackson,

I am quite sure that what you write is accurate, true. I'm not persuaded that either factoid is a good thing.