Mormonism and the nature of God/Trinity/Early beliefs
What is the historic church's concept of the Trinity?
Critics attempt to show that the LDS idea of deification is unbiblical, unchristian and untrue. They seem to think that this doctrine is the main reason why the LDS reject the Psychological Trinity. They write:
Why does the LDS Church reject the historic church's concept of the Trinity? Because not only does the Trinity remove any hope of a Mormon ever achieving godhood, but it also undermines Smith's first vision and subsequent teachings regarding a multiplicity of deities. If it can be demonstrated that the Father, Son, and Holy Ghost/Spirit are God, and at the same time be shown that there exists only one God, it would definitely place the integrity of the first Mormon prophet on the line.
It is important to remember that something like "the historic church's concept of the Trinity" does not exist. The doctrine of the Trinity as taught by Catholics and mainstream Protestants is rejected by Eastern Orthodoxy, and vice versa. The Trinity doctrine of the second century A.D. differs from that of the third, and the fourth century developed even newer ideas. Aquinas' doctrine is different from that of Augustine, and Barth again developed a new doctrine of the Trinity. Modern understanding of key terms like homoousios, person, nature, substance and essence don't have much to do with what they meant in the fourth century A.D.
Having said so much let me ask what the First Vision has to do with the Trinity? That's quite simple: Joseph Smith saw two personages, of which one was identified as the Father and the second as the Son. LDS since Joseph Smith have stated that Father, Son and Holy Ghost cannot be numerically one, if Joseph saw two personages. The authors believe that if the Trinity were right, then it would be impossible that Joseph Smith saw two personages, the Father and the Son. Here they clearly prove that they believe the Trinity teaches that Father, Son and Holy Ghost are numerically one, without any reservations. But, as hard as it is, that is not the doctrine of the Trinity. Just hear the words of Origen:
Now there are many who are sincerely concerned about religion, and who fall here into great perplexity. They are afraid that they may be proclaiming two Gods, and their fear drives them into doctrines which are false and wicked. Either they deny that the Son has a distinct nature of His own besides that of the Father, and make Him whom they call the Son to be God all but the name, or they deny the divinity of the Son, giving Him a separate existence of His own, and making His sphere of essence fall outside that of the Father, so that they are separable from each other. 
In trying to fight "Mormon heresy," they themselves promote the heresy of modalism, namely:
The Monarchians properly so-called (Modalists) exaggerated the oneness of the Father and the Son so as to make them but one Person; thus the distinctions in the Holy Trinity are energies or modes, not Persons: God the Father appears on earth as Son; hence it seemed to their opponents that Monarchians made the Father suffer and die. In the West they were called Patripassians, whereas in the East they are usually called Sabellians. The first to visit Rome was probably Praxeas, who went on to Carthage some time before 206-208; but he was apparently not in reality a heresiarch, and the arguments refuted by Tertullian somewhat later in his book "Adversus Praxean" are doubtless those of the Roman Monarchians (see PRAXEAS). 
- Let's refute this from the Bible:
Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and [to] my God, and your God. John 20:17
If the Father and the Son were numerically one, where would Jesus go?
Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear. Acts 2:33
If Father and Son and Holy Ghost were numerically one, how could Jesus be at the right hand of God? How would He be exalted? And why would He need to have received a promise of the Holy Ghost? Let's look at Jesus' own words:
And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me? Matthew 27:46
If the Father is the Son, then how could he have forsaken himself?
And last but not least, let me quote Augustine of Hippo, final framer of the Latin doctrine of the Trinity:
But under the oak at Mamre he saw three men, whom he invited, and hospitably received, and ministered to them as they feasted. Yet Scripture at the beginning of that narrative does not say, three men appeared to him, but, "The Lord appeared to him." And then, setting forth in due order after what manner the Lord appeared to him, it has added the account of the three men, whom Abraham invites to his hospitality in the plural number, and afterwards speaks to them in the singular number as one; and as one He promises him a son by Sara, viz. the one whom the Scripture calls Lord, as in the beginning of the same narrative, "The Lord," it says, "appeared to Abraham." He invites them then, and washes their feet, and leads them forth at their departure, as though they were men; but he speaks as with the Lord God, whether when a son is promised to him, or when the destruction is shown to him that was impending over Sodom. 
So even he, who like no other changed how a majority of Christians thinks of God, who, like none before him, elevated the oneness of God to a level where Orthodoxy felt it was only a semi-Sabellianistic distortion of the truth, he himself did not have a problem to conclude that God is numerically three persons, who can be seen in a vision or can visit a human being in the outer form of three male human beings simultaneously. So, if critics think the First Vision is crucial in showing Joseph's "error," we have to conclude that they are, in fact, even more modalistic than Augustine of Hippo. And more modalistic than "semi-modalistic" can only be fully modalistic.
What Does Deification (Theosis) Have To Do With It?
What does the doctrine of eternal progress, or deification have to do with the Trinity? It's quite easy to understand critic's false reasoning in that case, too: If the Trinity does not consist of multiple personages, which deserve to be called God, then how could we ever hope to become gods ourselves?
First of all, as we have shown so far, the premise is false: The Trinity, even the semi-Sabellianistic Psychological Trinity affirms three personages. And second, critics fail to understand that deification, or theosis, was the main doctrine of Christianity during the first centuries, it is still firmly taught in Orthodoxy, and even Catholicism has retained some belief in deification.
Let's see what the Bible has to say about the concept of deification:
Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. 1 John 3:2
What does the Protestant commentator Matthew Henry, quoted for his expertise on 2 Corinthians by critics, have to say about this verse?
The sons of God will be known and be made manifest by their likeness to their head: They shall be like him-like him in honour, and power, and glory. Their vile bodies shall be made like his glorious body; they shall be filled with life, light, and bliss from him. [note]
Wow! To be like God in honour, power and glory is a wonderful thing. That means to be placed above the angels. To be above anything, besides God, who will still be the "God of Gods." But it is not only Henry who teaches thus, and others have been far more blunt about our eternal destiny. Read the following quotes:
God on the one hand is Very God (Autotheos, God of Himself); and so the Saviour says in His prayer to the Father, "That they may know Thee the only true God; "but that all beyond the Very God is made God by participation in His divinity, and is not to be called simply God (with the article), but rather God (without article). And thus the first-born of all creation, who is the first to be with God, and to attract to Himself divinity, is a being of more exalted rank than the other gods beside Him, of whom God is the God, as it is written, "The God of gods, the Lord, hath spoken and called the earth." It was by the offices of the first-born that they became gods, for He drew from God in generous measure that they should be made gods, and He communicated it to them according to His own bounty. The true God, then, is "The God," and those who are formed after Him are gods, images, as it were, of Him the prototype. But the archetypal image, again, of all these images is the Word of God, who was in the beginning, and who by being with God is at all times God, not possessing that of Himself, but by His being with the Father, and not continuing to be God, if we should think of this, except by remaining always in uninterrupted contemplation of the depths of the Father. 
And Origen is not alone in this solemn claim.  Jordan Vajda, OP, a Dominican Catholic priest ("OP" stands for Ordo Praedicatorum-Order of Preachers-the official title of the Dominican order) even writes:
It seems that if one's soteriology cannot accommodate a doctrine of human divinization, then it has at least implicitly, if not explicitly, rejected the heritage of the early Christian church and departed from the faith of first millennium Christianity. However, if that is the case, those who would espouse such a soteriology also believe, in fact, that Christianity, from about the second century on, has apostatized and "gotten it wrong" on this core issue of human salvation. Thus, ironically, those who would excoriate Mormons for believing in the doctrine of exaltation actually agree with them that the early church experienced a "great apostasy" on fundamental doctrinal questions. And the supreme irony is that such persons should probably investigate the claims of the LDS Church, which proclaims that within itself is to be found the "restoration of all things. 
- [note] Origen, "Commentary on the Gospel of John," A Select Library of Nicene and Post-Nicene Fathers of the Church, Book II, 2.
- [note] "Modalism," Catholic Encyclopedia, http://www.newadvent.org/cathen/10448a.htm
- [note] Augustine of Hippo, "On The Trinity, Book II, Chapter 9" A Select Library of Nicene and Post-Nicene Fathers of the Church, Vol. VI.
- [note] "1 John," Matthew Henry's Commentary On The Whole Bible, Vol. VI, (Chester, 1721), as found at http://www.biblestudyguide.org/comment/matthew-henry/mh-complete/MHC00000.HTM
- [note] Origen, "Commentary on the Gospel of John, Book II, Chapter I," The Ante-Nicene Church Fathers.
- [note] St. Irenaeus, "Adv Haer III IV:38:4," The Ante-Nicene Church Fathers: "We are not made gods from the beginning; first we are mere humans, then we become gods." St. Maximus the Confessor : "Let us become the image of the one whole God, bearing nothing earthly in ourselves, so that we may consort with God and become gods, receiving from God our existence as gods." St. Athanasius, De inc.: "For the Son of God became man, that we might become God." St. Augustine: "He has called men gods that are deified of His Grace, not born of His Substance." St. Irenaeus, Adv Haer III: "The Word became flesh and the Son of God became the Son of Man: so that man, by entering into communion with the Word and thus receiving divine sonship, might become a son of God." St. Augustine of Hippo: "Let us applaud and give thanks that we have become not only Christians but Christ himself. Do you understand, my brothers, the grace that God our head has given us? Be filled with wonder and joy--we have become veritable Christs!" St. Thomas Aquinas: "The Only-begotten Son of God, wanting us to be partakers of his divinity, assumed our human nature so that, having become man, he might make men gods." St Basil the Great: "The highest of all things desired is to become God."
- [note] Jordan Vajda, "'Partakers of the Divine Nature': A Comparative Analysis of Patristic and Mormon Doctrines of Divinization" (master's thesis, Graduate Theological Union, 1998), 14