According to the Catholic Church, is belief in trinitarianism required for salvation?

Question

According to the Catholic Church, is belief in trinitarianism required for salvation?

Note: this is not asking whether trinitarianism is true, or whether a valid baptism is required, but whether one must believe in trinitarianism for salvation.

Answer 1

According to the Catholic Church, is belief in trinitarianism required for salvation?

Most certainly yes, as from even before the Nicene creed was formulated, candidates for baptism were required to answer several questions to profess belief in the Trinitarian God.

From today's Rite of Catholic Baptism (Parents and Godparents answer on behalf of the infant; same answers are expected for adult initiates):

...

CELEBRANT: Do you believe in God, the Father almighty, creator of heaven and earth?

PARENTS AND GODPARENTS: I do.

CELEBRANT: Do you believe in Jesus Christ, his only Son, our Lord, who was born of the Virgin Mary, was crucified, died, and was buried, rose from the dead, and is now seated at the right hand of the Father?

PARENTS AND GODPARENTS: I do.

CELEBRANT: Do you believe in the Holy Spirit, the holy catholic Church, the communion of saints, the forgiveness of sins, the resurrection of the body, and life everlasting?

PARENTS AND GODPARENTS: I do.

...

Also, the Catholic Catechism clearly teaches this:

232 Christians are baptized "in the name of the Father and of the Son and of the Holy Spirit"⁵³ Before receiving the sacrament, they respond to a three-part question when asked to confess the Father, the Son and the Spirit: "I do." "The faith of all Christians rests on the Trinity."⁵⁴

⁵³ Mt. 28:19

⁵⁴ St. Caesarius of Arles, Sermo 9, Exp. symb.:CCL 103,47.

Yes, but even if the wording of the a baptism involves holding to trinitarianism (I'm not sure it does given the wording above, TBH) most Catholics are baptized as infants, and so their beliefs aren't the ones being verified. (from comment)

Correct, but if we examine the whole context of the Rite of Baptism linked above, there is an expectation from the Parents to teach the child the Catholic faith:

CELEBRANT: You have asked to have your child baptized. In doing so you are accepting the responsibility of training him (her) in the practice of the faith. It will be your duty to bring him (her) up to keep God's commandments as Christ taught us, by loving God and our neighbor. Do you clearly understand what you are undertaking?

PARENTS: We do.

Therefore, when the child is old enough to take first communion, a common way for Catholic parents to discharge their promise is to give the child a mini catechism such as what this article describes using a popular text such as Saint Joseph First Communion catechism containing clear teaching of the Trinity suitable for 1st-2nd grade kids.

From Lesson 3 titled "The Blessed Trinity":

10. Is there only one God?

Yes, there is only one God.

 There cannot be more than one God.
 There never will be more than one God.

11. How many Persons are there in God?

In God there are three Persons -- the Father, the Son, and the Holy Ghost

 The Father is God.
 The Son is God.
 The Holy Ghost is God.
 But they are all ONE God.
 The Father is the First Person.
 The Son is the Second Person.
 The Holy Ghost is the Third Person.
 There are THREE Persons in ONE God.

12. What do we call the three Persons in one God?

We call the three Persons in one God the Blessed Trinity.

[there is even a picture, see page 16]

13. How do we know that there are three Persons in one God?

We know that there are three Persons in one God because we have God's word for it.

 The Father sent His Son to us on earth
 The Son told us what the Blessed Trinity is like.
 The Blessed Trinity is God's family.
 But it is a family which is all one God.
 In this family all is love.
 They never fight.
 They always agree.
 They are always happy.

 Say this prayer often: O God, make my family holy.

Can you clarify how these aspects of baptism impinge on the question of whether one must believe in trinitarianism for salvation? (from comment)

I understand that as adults we can ask theologically and philosophically complicated questions and I do appreciate how great Christian theologians cum philosophers like St. Thomas Aquinas can develop whole systems of thoughts to plumb the depth of the Trinitarian doctrine from God's side (as mystery) and from our side (through reason).

But when it comes to salvation it is a highly PRACTICAL matter to undertake the LIFE of faith through the sacraments of the church, and in order to fully incorporate those sacraments (the primary being the Eucharist) into someone's spiritual life, the most basic assumption about God needs to be there for those sacraments to even make sense. For example: if Jesus is not in essence God, the Catholic eucharist rite is tantamount to idolatry! Hence the custom of First Communion catechism that I deliberately quoted at length above to show how even the most uneducated person should have no problem understanding it.

As you can see in the Catholic Order of Mass there is the reciting of the creed (step 18) prior to the Communion Rite (starting at step 124). The expectation is to prepare our hearts to receive communion properly, which includes regarding the host to be the real presence of Jesus. If we do not believe in Trinitarianism, then taking communion would amount to sacrilege which is mortal sin. Reciting the creed with doubts is okay as long as we give the church a chance to explain.

Similarly, even someone baptized as an adult might change their views. (from comment)

The moment a baptized Catholic deliberately refuses to believe that God is Trinity, in the eyes of the Catholic Church, the person has committed the sin of heresy or apostasy. See my answer to another question. As I explained in the answer, there is a big difference between having doubts and acting on them. It is fine asking difficult questions about the Trinity (as I do myself), but while asking those questions, the PRACTICAL matter (the one that the Catholic Church sees as very important to shepherd her faithful) is whether the spiritual LIFE is healthy. She does it through issuing and enforcing practical regulations on church practice.

Answer 2

St. Thomas Aquinas addresses exactly this question in Summa Theologica II-II q. 2 a. 8 ("Whether it is necessary for salvation to believe explicitly in the Trinity?"), answering (co.):

It is impossible to believe explicitly in the mystery of Christ, without faith in the Trinity, since the mystery of Christ includes that the Son of God took flesh; that He renewed the world through the grace of the Holy Ghost; and again, that He was conceived by the Holy Ghost. Wherefore just as, before Christ, the mystery of Christ was believed explicitly by the learned [e.g., Abraham and Moses*], but implicitly and under a veil, so to speak, by the simple, so too was it with the mystery of the Trinity. And consequently, when once grace had been revealed, all were bound to explicit faith in the mystery of the Trinity: and all who are born again in Christ, have this bestowed on them by the invocation of the Trinity, according to Mt. 28:19: "Going therefore teach ye all nations, baptizing them in the name of the Father, and of the Son and of the Holy Ghost."

^{*A fortiori, St. Paul knew the doctrine of the Trinity.}

Answer 3

The Trinity is a Dogma of the Catholic Church

1 THE creed which is called Apostolic is composed essentially of (1) a Trinitarian part, three articles professing faith in three divine persons; (2) a Christological part which was added to the first section.

Denzinger

All Dogmas of the Catholic Church must have the full "obedience of faith" by Catholics

"The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful - who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals... . The infallibility promised to the Church is also present in the body of bishops when, together with Peter's successor, they exercise the supreme Magisterium," above all in an Ecumenical Council. When the Church through its supreme Magisterium proposes a doctrine "for belief as being divinely revealed," and as the teaching of Christ, the definitions "must be adhered to with the obedience of faith." This infallibility extends as far as the deposit of divine Revelation itself.

Catechism of the Catholic Church 891

There is no salvation outside the Catholic Church

If someone from this people wants to be saved, let him come into this house so that he may be able to attain his salvation. . . . Let no one, then, be persuaded otherwise, nor let anyone deceive himself: Outside of this house, that is, outside of the Church, no one is saved; for, if anyone should go out of it, he is guilty of his own death

Origen

Therefore all Catholics must believe in the Trinity to be saved.

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HOWEVER, and not exactly pertinent to the question, but worth noting, what the meaning of No Salvation outside the Church is. It doesn't necessarily mean everyone else is damned, it just means that if you are saved, it'll be by Christ's saving grace and "The grace necessary for salvation continues to come from Christ, through his Church."

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Lastly, check out the Athanasian Creed

He therefore that will be saved, must thus think of the Trinity.