The Roman Catholic Church does not recognize, in a doctrinal sense (obviously, it does in a secular material sense) the existence of any other Christian Churches [0]; it takes the “Catholic” in its name quite seriously.

From the doctrinal point of view, there are no non-Catholic Christians, just Catholics of various degrees of imperfection, is and everyone who intends to join Christianity, or to join someone else to it, intends joining to the Catholic Church as the two are identical and coextensive.

[0] it does recognize particular churches, but they are part of rather than separate from the universal Church; in addition to those in full communion with Rome, these include Christian communities who are out of Communion with Rome but who retain valid episcopal apostolic succession, including the various Eastern and Oriental Orthodox Churches, among others.

> Generally Protestants don't recognize the authority of the church at all (i.e. Sola scriptura).

Sola scriptura is a doctrine of a subset of protestantism; there are Protestants who don't adhere to it but instead to prima scriptura, or to doctrines that aren't clearly either. It is certainly not the case that Protestants as a whole reject ecclesiastical authority.

> it takes the “Catholic” in its name quite seriously.

I'm pretty sure you know about this, but for laymen who may not be familiar with the terminology.

The word "catholic" (little c) literally means "universal". And it is a strong doctrine of the "Catholic church" (big C) that the group of worshipers is singular and universal.

It's not clear to me whether Rome accepts Christians who completely reject its authority. The central point of Lutheran Protestantism is that it's open source: anybody with a Bible can decide for themselves how to implement it without asking for permission. Of course there's value in a community of believers but the requirement is just to gather "two or more".

I guess Protestants have a harder time deciding who's a Christian; probably acceptance of the Nicean Creed is a popular standard, though I prefer "anyone who seeks to imitate Christ".

> It's not clear to me whether Rome accepts Christians who completely reject its authority

Rome absolutely accepts them as Christians.

It views them as wrong, but it does that for plenty of Christians that don't completely reject it's authority, too.

> The central point of Lutheran Protestantism is that it's open source: anybody with a Bible can decide for themselves how to implement it without asking for permission.

It would seem to me that the central point of Lutheran Protestantism is that there is a right way to implement Christianity, that right way is clear in Scripture, and having intermediaries between the believer and the Scripture is a barrier to the believer finding the truth in Scripture.

Lutheranism also seems generally pretty strongly defined by a particular opinion on what the right answers are, or at least a wide range of them, though somewhat internally divided by the degree of certainty ascribed to those answers.

Thank you, just to be clear: there is no obstacle to a Protestant sponsoring a child's baptism in the Catholic Church as parent or godparent?

Also I think you are right that many Protestants believe their opinions are clearly implicated by scripture, but it should be obvious on reflection that Scripture is not self-interpreting.

> Thank you, just to be clear: there is no obstacle to a Protestant sponsoring a child's baptism in the Catholic Church as parent or godparent?

Strictly speaking, there is a distinction between "sponsor" and "witness". The sponsor must be a Catholic, the witnesses can be non-Catholic Christians. However, in my personal experience (having had one child baptised Catholic, and the other will be baptised soon), everyone just collectively treats the "sponsor" and "witnesses" as "godparents". The parish just tells people "one godparent must be a Catholic". The sponsor is just the first godparent listed on the baptism certificate.

As far as parents go, technically there is no requirement that either parent be Catholic, or even Christian. A Buddhist couple could ask for their child to be baptised as Catholic. Now, in practice, priests will be extremely hesitant about fulfilling such a request, but strictly speaking it is allowed.

My understanding is there needs to be a reasonable expectation the child would be raised to be Catholic. A typical heuristic is at least one Catholic parent. I guess "be Catholic" in this context means "will go through confirmation" or something to that effect.

Yes. A priest is not supposed to baptise without such a "reasonable expectation". (Although if he does it anyway, he has broken the rules, and could be disciplined for it, but the baptism is nonetheless valid.)

One Catholic parent is the usual heuristic but it isn't actually the rule so isn't absolutely binding. For example, if a non-Catholic is known to regularly attend Catholic services, but for whatever reason is hesitating in formally converting, a priest may very well agree to such a parent's request to baptise their child.

Donatists claimed that sacraments were only valid if the priest who performed them was a faithful Christian. So if the priest was a heretic or grave sinner, the sacraments he performed were invalid. The Catholic Church declared that to be a heresy – a heretic or a sinner can still perform valid sacraments, provided the heresy doesn't involve a radically different understanding of the nature of the sacrament.

So, from the viewpoint of traditional Catholic theology, Protestants are heretics, but their beliefs about baptism are close enough to the Catholic belief that their heresy doesn't invalidate their baptisms. (Different story for ordination and the eucharist, since Protestant theology on those topics is further away from Catholic theology.) Whereas, non-Trinitarian Christians, the Catholic Church views their beliefs to directly touch on what the sacrament of baptism is about, so their non-Trinitarianism invalidates their baptisms from the Catholic viewpoint.

> So, from the viewpoint of traditional Catholic theology, Protestants are heretics, but their beliefs about baptism are close enough to the Catholic belief that their heresy doesn't invalidate their baptisms. (Different story for ordination and the eucharist, since Protestant theology on those topics is further away from Catholic theology.)

This is largely an issue of apostolic succession; regardless of the theology of the communities about ordination and the eucharist (which do differ critically), the sacraments cannot be valid because they are valid, in Catholic doctrine, only when performed by a validly ordained bishop (ordination) or consecrated by a validly ordained priest (Eucharist); the absence of episcopal apostolic succession (or, in much of protestantism, even the episcopal office) means that there are no validly ordained bishops and priests around. Now that is intimately tied to the doctrine around ordination, but even if the doctrine around the substance of the sacrament of the eucharist were identical, the absence of a valid minister of the sacrament would be critical.

> Whereas, non-Trinitarian Christians, the Catholic Church views their beliefs to directly touch on what the sacrament of baptism is about, so their non-Trinitarianism invalidates their baptisms from the Catholic viewpoint.

Well, there's two issues there. Sure, it's debatable whether a non-Trinitarian would have the requisite intent to validly (if, necessarily, illicitly) perform a baptism in the eyes of the Catholic Church, but what is more certainly fatal of most baptism by non-Trinitarians would be the failure to baptise using a Trinitarian formula, which is required for the sacrament to be valid, whether or not it is licit.

https://en.m.wikipedia.org/wiki/Valid_but_illicit

Thanks. I agree with most of what you say.

> This is largely an issue of apostolic succession

There are two different cases here. Protestants differ in their ecclesiology; some retain belief in episcopal apostolic succession, others reject it. Of course, those Protestants who reject episcopal apostolic succession, there are (from the Catholic viewpoint) no valid bishops, and hence no valid priests, and hence no valid Eucharist. On the other hand, the Anglicans, and some (but not all) of the Lutheran churches, claim to have bishops with valid apostolic succession. In the case of the Anglicans, the Catholic Church has decided that their eucharistic theology is so different that their episcopal consecrations and ordinations are invalid as a result (Apostolicae curae, 1896). Presumably, the same judgement would apply to other Protestant denominations claiming episcopal apostolic succession also.

I see, but in addition to a valid baptism aren't there required beliefs as well? Such as the immaculate conception and Papal infallibility, which Protestants would deny?

According to Catholic teaching, any validly baptised person is a Christian, regardless of their beliefs. Baptism is an indelible mark on the soul, like a spiritual membership card that you cannot destroy. Once you join, you can never really leave. Even if you convert to Islam, or become an atheist, you remain (in some spiritual sense) a Christian as long as you live. This is very different from the Protestant understanding of what it means "to be a Christian", which is based on a person's belief and (more importantly) the inner attitude of their heart.