According to Reformed theology, if one believes that Yeshua (Jesus) is the Messiah and the son of God (but they deny that he is God and the second person of a triune god), is this person sent to an eternal hell where they will have their flesh burned forever and ever in agonizing pain?
Yes (in as much as any human can know who is sent to hell). According to the Athanasian Creed (one of the three creeds which the majority of Reformed churches hold to), to be saved, you must believe in the Trinity as set out in said creed. If you are not saved, you will go to hell.
The vast majority of Reformed churches uphold the Athanasian, either through their subscribing to the Three Forms of Unity, which includes the Belgic Confession, which directly states that the Athanasian Creed's teaching on the Trinity is true and correct, or through directly stating that their official doctrines and statements of faith include the Athanasian Creed.
I add the caveat about human knowledge because it is certain that no one person has all knowledge or truth in their doctrine; as such, who can know at what exact flaw in doctrine someone is not saved? But certainly, according to the official creeds and confessions of the Reformed church, non-trinitarians go to hell.
I can provide additional sources if necessary, but the Athanasian Creed is fundamental and lays it out very clearly.
Reformed theology has indeed, generally speaking, affirmed that those who reject the doctrine of the Trinity are not saved and therefore go to hell.1 Birdie's answer to this question mentions the Belgic Confession, a highly regarded reformed confession, which reads:
This doctrine of the holy Trinity has always been maintained in the true church, from the time of the apostles until the present, against Jews, Muslims, and certain false Christians and heretics, such as Marcion, Mani, Praxeas, Sabellius, Paul of Samosata, Arius, and others like them, who were rightly condemned by the holy fathers. And so, in this matter we willingly accept the three ecumenical creeds—the Apostles’, Nicene, and Athanasian—as well as what the ancient fathers decided in agreement with them.2
One of the more infamous portions of the Athanasian Creed is particularly relevant:
Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except every one do keep whole and undefiled; without doubt he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity [...] the Unity in Trinity, and the Trinity in Unity, is to be worshipped. He therefore that will be saved, let him thus think of the Trinity.
Though direct affirmation of the Athanasian Creed is not found in other reformed creeds, like the Westminster Standards or the Baptist Confession of Faith, many Reformed theologians have expressed their agreement with the Athanasian Creed, including the "damnatory clauses."3
However, it's nonetheless important to note the care with which many Reformed thinkers apply the language of the creed. Philip Schaff expresses general discomfort with damnatory clauses in creeds, and argues for a distinction between those who lack full knowledge of the Trinity and those who reject the truth of it:
This threefold anathema, in its natural historical sense, is not merely a solemn warning against the great danger of heresy, nor, on the other hand, does it demand, as a condition of salvation, a full knowledge of, and assent to, the logical statement of the doctrines set forth (for this would condemn the great mass even of Christian believers); but it does mean to exclude from heaven all who reject the divine truth therein taught. It requires every one who would be saved to believe in the only true and living God, Father, Son, and Holy Ghost, one in essence, three in persons, and in one Jesus Christ, very God and very Man in one person.4
John Newton approaches the language similarly, suggesting that imperfect knowledge does not damn:
I believe with you, that a man may be saved who never heard of the creed, who never read any book but the New Testament, or perhaps a single evangelist; but he must be taught of God the things that accompany salvation, or I do not think he can be saved.5
Finally, some express even more caution, such as Ra McLaughlin, while still affirming the general point:
I would hope to see great tolerance for those ignorant/agnostic in these areas. A lack of understanding of these doctrines deprives one of rich opportunities for growth and worship, but it does not threaten souls. However, there can be great danger in holding to misconceived ideas about these matters. For example, those who reject the deity and/or humanity of Christ cannot be saved (John 20:30-31; 1 John 2:22; 4:2-3; 2 John 7). In this regard I differentiate rather significantly between passive ignorance and active rejection.6
Reformed thinkers widely accept the content of the "damnatory clauses" of the Athanasian Creed, either explicitly or implicitly (by not rejecting or qualifying it while discussing the Athanasian creed). This should not be taken to mean, however, that they believe that only those who affirm and understand the creed (and fully understand the doctrine of the Trinity) can be saved. Many explicitly indicate that those with imperfect knowledge of the doctrine can still be saved, while regarding those who actively reject the Trinity as worshiping the wrong God. As Michael Reeves writes:
Which God we worship: that is the article of faith that stands before all others. The bedrock of our faith is nothing less than God himself, and every aspect of the gospel—creation, revelation, salvation—is only Christian insofar as it is the creation, revelation and salvation of this God, the triune God. I could believe in the death of a man called Jesus, I could believe in his bodily resurrection, I could even believe in a salvation by grace alone; but if I do not believe in this God, then, quite simply, I am not a Christian.7
References and notes:
- Note: Dealing with the exact definition of hell held by each of the theologians quoted here would be excessively complex and would distract from the main point. However, it's important to note that the long-standing majority view in Reformed theology is that hell is a place of eternal torment. The "flesh burning" aspect will often be rejected as figurative language for severe suffering more generally (including spiritual suffering). And some associated with Reformed theology hold to universalism and annihilationism.
- Belgic Confession
- See Joshua Wilson, English Presbyterian Chapels, 7ff. Wilson quotes a number of historical Reformed theologians who accepted at least the theological content of the damnatory clauses, if not their historical connection to the Athanasian Creed. But many of the quotes indicate a concurrence with the other authors quoted here—Richard Baxter, for example, writes that "faith in the three articles of the baptismal covenant makes us Christians" (8) – a very simple test that does not require an extensive knowledge of the doctrine of the Trinity nor of the Athanasian Creed's language.
- Schaff, Creeds of Christendom, I
- Newton, Letter 1 to Rev. Mr. S., June 23, 1775, in Works
- McLaughlin, Does the Doctrine of the Trinity Matter?
- Reeves, Delighting in the Trinity, 15–16