According to Protestants who hold that belief in Trinitarianism is necessary for salvation, what happens to those who assent but don't understand?

Salvation comes from repentance of sins and faith. This question deals with the latter. We could ask ourselves, faith in what or in whom? Let's look at Acts 20:21

Testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ.

The goal of faith is our Lord Jesus Christ. So it is not about a general vision of God, in which you have to have faith, but something specific. It is believing in Christ as our Lord and Saviour, our Redeemer, our Propitiation, our Reconciler. It is believing in Him as the only one with power and authority to save us, because He is God. He is not a god. But, the only living and true God.

So, if He is God, necessarily the doctrine of the Trinity is a requirement for salvation, because Jesus is refered as God, as well as the Father and the Holy Spirit. You must recognize this, even if you cannot fully explain it. Knowing that no one can really explain God, be it the Trinity or any of His attributes, since He surpasses our understanding. We can grow in the knowledge of God, but we will never be experts.

To reject the full humanity of Christ or His full deity is to change the gospel, thereby making salvation impossible. Anyone who flatly denies these truths would be considered a heretic and therefore not a Christian.

26 And he said, “The kingdom of God is as if a man should scatter seed on the ground. 27 He sleeps and rises night and day, and the seed sprouts and grows; he knows not how. 28 The earth produces by itself, first the blade, then the ear, then the full grain in the ear. 29 But when the grain is ripe, at once he puts in the sickle, because the harvest has come.”

I have clear recollections of the time when I was a new believer. I was asked by my Bible study leader to memorize this passage in Galatians 2:

20 I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. 21 I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.

This verse is about receiving a new identity in Christ, of dying to the old self. I had no clue at the time what it meant. One month later, I was freed from the fear of death and the fear of living a worthless life of no consequence. I attribute that change in me to those verses. The Holy Spirit made words I did not understand grow inside me, though I did not know how, just like the farmer in Jesus' parable.

I assume that assent to the Doctrine of the Trinity is the same. The Holy Spirit can lead an ignorant but faithful Christian into all truth, though it may take time. Understanding can produce faith, but the walk of faith can also produce understanding. Sometimes the Word must change us before we can understand what it means.

In the end, it is not assent to the doctrine of the Trinity that saves you. What saves you is embracing each member of the Trinity as they visit you. The doctrine is the light by which you can understand each visit. I remember crying at the thought that Jesus died on the cross for me personally. That was when I knew that Jesus was a real person, because I could not cry for an abstraction. Likewise, when I visited a Christian woman on her deathbed out of obedience, the Holy Spirit overpowered me and delivered me from a decade of depression. I knew that it was the Holy Spirit because I had been taught that there was a Holy Spirit and Joy one of its fruits. Finally, when in a dream I was warned of years of suffering about to begin and later struck deaf in one ear, then healed after reading a passage in Exodus about how the Father can cause deafness and sternly directed Moses to go to Egypt, I realized I had met with the Father.

These are not my only encounters with each member of the Trinity, but they are the most potent. The doctrines prepare you to recognize God however He chooses to appear to you. It is up to you to embrace God, whether as Father, Son, or Holy Spirit, or all three at once in an undifferentiated unity. It is that embrace, obedience, trust, and acceptance of God when He appears as He chooses to make Himself known that counts. That is what saves.

When Joshua met the man with the drawn sword in Joshua 5, why did he accept the man's declaration that he was the commander of the host of the Lord? It is because of a worship song in Deuteronomy 32 that Moses spoke to the people. Moses said that God would draw his sword and fight for his people, but they must take the words of the song to heart. That passage makes a big deal out of the fact that Joshua was present for the singing of that song. Joshua remembered the song and therefore recognized God on the day He came to visit him. That is what the doctrine of the Trinity does for us: it helps us recognize God when he visits us, so that we may not be deceived by counterfeits.

What "Protestants hold that belief in Trinitarianism is required for salvation"? My understanding of Protestantism is that salvation comes only because of divine grace or "unmerited favor" (sola gratia).

I suppose that Jehovah's Witnesses and Swedenborgians and Oneness Pentecostals are not considered Protestant by some mainline denominations, but I think they consider themselves Protestant.

But I know of no Protestants that insist that you must believe exactly as they do to receive God's love and salvation from your own sin and corruption. Even those Protestants that I know that are, themselves, strict Trinitarians. Some believe that Modalism is a heresy, but ya gotta be careful condemning heretics (they might be closer to God than you).

The question gives as an example of the difficulty, "I can say "Jesus is God," but the 'is' there is ambiguous. It could be taken in all sorts of senses, one of which is the 'official' Trinitarian sense."

This shows a common misunderstand many people who don't understand the Trinity doctrine have. Not understanding the doctrine, however, is not the critical issue, but misrepresenting it to others is what causes confusion. The Bible states that the Word, who was with God in the beginning, and who is God, and who made everything that was made, was made flesh - John 1:1-14. This has to be believed even though it cannot be explained in words outside of the Bible. This is demonstrated by doubting Thomas exclaiming before the risen Christ, "My Lord and my God!" (John 20:28). Christ accepted that declaration of faith and worship. But it's doubtful if Thomas could have explained it in a way that people today look for it to be explained.

The point here is that those who do not have the belief in Christ being the Son of God, that John's gospel was all about (Jn.20:31), would wrongly suppose that his opening words could be taken to mean that this One who became flesh was "a god", or was "divine". Those are two examples of ambiguity with the Bible saying the Word was God. A lesser (seconary) god, or some kind of divinity is supposed. Yet the Bible disallows that, which is what the Trinity doctrine is careful to state. When it comes to the Trinity doctrine, it is more about saying what the Godhead is not, than what it is! This surprises a lot of people, but there is wisdom in that, for no mere human can ever fully explain God. Yet God has provided us with enough revelation of himself, through the person of Jesus Christ and the working of the Holy Spirit, to enable even the newest, least-learned Christian to kneel down and proclaim of the risen Christ, that he is their Lord and their God.

That's a basic point of the Trinity doctrine. Believe that, and you are in mainstream, orthodoxly Christian belief. You might not be able to explain it, but that's not what anybody's salvation depends upon. It is faith in who this Son of God is - the only One who has dealt with our sin so that God can forgive us - that is vital - Matthew 16:15-17 - and it comes with divine revelation, as Peter discovered there (and which Catholics, Orthodox, and Protestants are agreed on.) Theology can explain why it had to be God incarnate who became flesh and died for righteous judgment on sin to be poured out, but that's another question. To begin with, a Christian must bend their knees in Jesus' name and confess him to be who the Bible says he is (Acts 2:21-39) - not what some groups claim him to be - a sinless man, a great example, but a creature nonetheless. That is what the Trinity doctrine protects from.

It's belief in Jesus Christ as being Lord and God that saves, whether or not we can explain that to others. Those who genuinely assent without really understanding can confidently study the scriptures to have understanding, for the Holy Spirit teaches such ones (John 15:26 & 16:13).

It is generally acknowledged that the precise nature of God is beyond human comprehension. God is infinite, and all humans are limited, fallible beings. He is much more than our limited understanding can comprehend (Job 36:6, Job 37:5, Isaiah 40:28). We can have some understanding of God, but a complete understanding is beyond us.

There is, in my experience, no minimum level of understanding that is required. I know of many people in many churches whose position is "I don't really understand it, but I believe it". No church leader I am aware of has ever criticized this position. I have known many people of simple understanding who have come to faith, and many of the intellectual concepts of Christianity are beyond them. Nobody I know has ever sought to exclude them on that basis. Doing so would appear to be completely contrary to the teachings of Jesus, who taught that faith and not intellectual rigour was the cornerstone of salvation, and who welcomed children into the Kingdom of God.

While the subtleties of the Trinity are rarely gone into, the essence is usually taught clearly in churches:

• Jesus is God
• The Father is God
• The Holy Spirit is God
• These three together are one God
• Jesus is also fully human.

Most Christians would understand all of those points.