As has been noted by many, functionally, many Trinitarians, both historical and modern, are Nestorian, often treating Jesus as two persons. Furthermore, when it comes to the true and full humanity of Jesus, Trinitarian dogma tends to pay lip service to such. On this, and how Latter-day Saint Christology is in a better position, see:
Latter-day Saints have Chosen the True, Biblical Jesus.
The Christological Necessity of Universal Pre-Existence
One 19th century Unitarian critic of Trinitarian theology on the issue of “how many” God is (e.g., the charge of Tritheism) and the Hypostatic Union, wrote the following which shows the “humpty dumpty” nature of language used by Trinitarians:
It is constantly affirmed that the doctrines of the Trinity, and of the two natures in Christ, comprise no contradiction; that it is not stated in the former that there are three Gods, but that God is three in one sense, and one in another; and in the latter, that Christ is two in one sense, and one in another.
I repeat and proceed to justify my statement, that if, in the enunciation of these tenets, language is used with any appreciable meaning, they are contradictions; and if not, they are senseless. I enter upon this miserable logomachy with the utmost repugnance; and am ashamed that in vindication of the simplicity of Christ, we should be dragged back into the barren conflicts of the schools.
“If,” says Dr. Tattershall, “it had been said that He is ONE God and also THREE GODS, then the statement would have been self-contradictory, and no evidence could have established the truth of such a proposition” (Sermon on the Integrity of the Canon, p. 80). Now I take it as admitted that this being is called ONE GOD; and that there are THREE GODS, is undoubtedly affirmed distributively, though not collectively; each of the three persons being separately announced as God. In the successive instances, which we are warned to keep distinct, and not confound, of the Father, the Son, and the Holy Ghost, proper Deity is affirmed; in three separate cases, all that is requisite to constitute the proper notion of God, is said to exist; and this is exactly what is meant, and all that can be meant, by the statement, that there are three Gods. I submit then that the same creed teaches that there are three Gods, and also that there are not three Gods.
From this contradiction there is but one escape, and that is, by declaring that the word of God is used in different senses; being applied to the triad in one meaning, and to the persons in another. If this be alleged, I wait to be informed of the new signification which is to be attached to this title, hitherto expressive of all the ideas I can form of intellectual and moral perfection. More than this, which exhausts all the resources of my thought, it cannot mean; and if it is to mean less, then it withholds from Him to whom it is applied something which I have hitherto esteemed as essential to God. Meanwhile, a word with an occult meaning is a word with no meaning; and the preposition containing it is altogether senseless.
But the favourite way of propounding this doctrine is the following: that God is three in one sense, and one in another; Three in Person, but only One Individual, Subsistence, or Being. The sense, then, if I understand aright of the word person, is different from the sense of the words Individual, Being, or Subsistence; and if so, I may ask what the respective senses are, and wherein they differ from each other. In reply I am assured, that by person is to be understood “a subject in which resides” “an entire set or series of those properties which are understood to constitute personality; viz. the property of Lie, that of Intelligence, that of Volition, and that of Activity, or power of Action (Dr. Tattershall’s Sermon on the Integrity of the Canon, p. 81). Very well; this is distinct and satisfactory; and now for the other sense, viz. of the words Individual, Being, and Subsistence. About this an ominous silence is observed; and all information is withheld respecting the quite different meaning which these terms contain. Now I say, that their signification is the very same with that of the word Person, as above defined; that when you have enumerated to me a complete “set of personal attributes,” you have called up the idea of an Individual, Being, or Subsistence; and that when you have mentioned to me these phrases, you have made me think of a complete set of personal attributes; that if you introduce me to two or three series of personal attributes, you force me to conceive of two or three beings; that a complete set of properties makes up an entire subsistence, and that an entire subsistence contains nothing else than its aggregate of properties. To take, for example, from Tattershall’s list of qualities which are essential to personality; tell me of two lives, and I cannot but think of two individuals; of two intelligences, and I am necessitated to conceive of two intelligent beings; of two wills or powers of action, and it is impossible to restrain me from the idea of two Agents; and if each of these lives, intelligences, and volitions, be divine, of two Gods. The word substance, in fact, will hold no more than the word person; and to the mind, though not to the ear, the announcement in question really is, that there are three persons, and yet only one person. Thus men “slide insensibly,” to use the words of Archbishop Whately, “into the unthought-of, but, I fear, not uncommon, error of Tritheism; from which they think themselves the more secure, because they always maintain the Unity of the Deity; though they gradually come to understand that Unity in a merely figurative sense; viz. as a Unity of substance,--a Unity of purpose, concern of action, &c.; just as any one commonly says, ‘My friend such-an-one and myself are one;’ meaning that they pursue the same designs with entire mutual confidence, and perfect co-operation, and have that exact agreement in opinions, views, tastes, &c., which is often denoted by the expression one mind” (Elements of Logic. Appendix, in verb. Person).
No doubt this excellent writer is correct in his impression, that the belief in three Gods is prevalent in this country, and kept alive by the creeds of his own church. And how does he avoid this consequence himself? By understanding the word Persons, not in Dr. Tattershall’s, which is the ordinary English sense, but in the Latin signification, to denote the relations, or capacities, or characters, which an individual may sustain, the several parts which he may perform; so that the doctrine of the Trinity amounts only to this, that the One Infinite Deity bears three relations to us. This is plain Unitarianism, veiled behind the thinnest disguise of speech. Between this and Tritheism, it is vain to seek for any third estate.
The contradiction involved in the doctrine of the two natures of Christ is of precisely the same nature and extent. We are assured that he had a perfect human constitution, consisting of the growing body ad progressing mind of a man; and also a proper divine personality, comprising all the attributes of God. Now, during this conjunction, either the human mind within him was, or it was not, conscious of the co-existence and operation of the divine. If it was not, if the earthly and celestial intelligence dwelt together in the same body without mutual recognition, like two persons enclosed in the same dark chamber, in ignorance of each other, then were there two distinct beings, whom it is a mockery to call “one Christ;” the humanity of our Lord was unaffected by his Deity, and in all respects the same as if disjoined from it; and his person was but a movable sign, indicating the place and presence of a God, who was as much foreign to him as to any other human being. If the human nature had a joint consciousness with the divine, then nothing can be affirmed of his humanity separately; and from his sorrows, his doubts, his prayers, his temptations, his death, every trace of reality vanish away. If he were conscious, in any sense, of omnipotence, nothing but duplicity could make him say, “of mine own self I can do nothing;” if of omniscience, I was mere deception to affirm that he was ignorant of the time of his second advent; if of his equality with the Father, it was a quibble to say, “my Father is greater than I.” I reject this hypothesis with unmitigated abhorrence, as involving in utter ruin the character of the most perfect of created beings.
The intrinsic incredibility then of these doctrines, involving, as they do, “clear immoralities and self-contradictions,” would throw discredit on the claims of any work professing to reveal them on the authority of God. And whether we listen to the demands of Scripture on our reverential attention, must depend on this:--whether these tenets are found there nor not. And to this enquiry let us now proceed.
One remark I would make in passing, on the supposed value of the theory of the two natures, as a key to unlock certain difficult passages of the Bible, and to reconcile their apparent contradictions. Christ, it is affirmed, is sometimes spoken of as possessing human qualities, sometimes as possessing divine; on the supposition of his being simply man, one class of these passages contradicts us; on the assumption of his being simply God, another. Let us then pronounce him both, and everything is set right; every part of the document becomes clear and intelligible.
Now which, let me ask, is the greater difficulty: the obscure language, which we wish to make consistent, or the prodigious hypothesis, devised for the reconcilement of its parts? The sole perplexity in these portions of Scripture consists in this,--that the divine and the human nature are felt to be incompatible, and not to be predicable of the same being: if we did not feel this, we should be conscious of no opposition; and the ingenious device for relieving the bewilderment, is to deny the incompatibility, and boldly to affirm the union. If you will but believe both sides of the contradiction, you will find the contradiction disappear! What would be thought of such a principle of interpretation applied to similar cases of verbal discrepancy? It is stated, for example, in the Book of Genesis, that Abraham and Lot received a divine communication respecting the destruction of Sodom; and the bearers of the message are spoken of, in one place, as Jehovah himself; in another, as angels; in a third as men (Genesis, xviii. 1, 2, 22; xix. 1, 10, 15). What attention would be given to any interpreter who should say; ‘it is clear that these persons could not be simply God, for they are called men; nor simply men, for they are called angels; nor simply angels, for they are called God: they must have had a triple nature, and been at the same time perfect God, perfect angel, and perfect man?’ Would such an explanation be felt to solve anything? Or take one other sense, in which Moses is called God with a distinctness which cannot be equalled in the case of Christ: “Moses called together all Israel, and said to them: . . . I have led you forty years in the wilderness; your clothes have not waxen old upon you and thy shoe is not waxen old upon thy foot. Ye have not eaten bread, neither have ye drunk wine or strong drink; that ye might know that I am the Lord your God” (Deut. xxix. 2, 5, 6). What relief, let me ask, should be obtained from the difficulty of this passage, by being told that Moses had two natures in one person, and must be received as God-man? Who would accept “a key” like this, and not feel that in loosening one difficulty, it locked fast another, and left us in labyrinthine darkness? (James Martineau, “The proposition ‘That Christ is God,’ Proved to be False from the Jewish and the Christian Scriptures,” pp. 13-18, in Unitarianism Defended: A Series of Lectures By Three Protestant Dissenting Ministers of Liverpool: In Reply to A Course of Lectures, Entitled “Unitarianism Confuted,” By Thirteen Clergymen of the Church of England [Liverpool: Wilmer and Smith, 1839], 303-308)