John 17:3 is a rather problematic text for those who hold to the Trinity dogma. As I wrote in response to a Reformed Baptist critic of LDS Christology:
In the Bible we see that eternal life comes from knowing Jesus.
John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Already, Gilpin has used a text that refutes, not supports, his Christology Why? Firstly, one should note that in Trinitarian theology, there is an allowance (albeit, ambiguously) for a distinction between the “persons” of the Godhead (the Father is not the Son; the Son is not the Spirit; the Spirit is not the Father), as modalism would arise if no distinction was permitted between them; however, there is no allowance for a distinction between “God” or any of the divine titles (e.g., Yahweh; Adonai) and the persons, that is, the Father is “God” but so is the Son and Spirit. However, in many key “creedal” texts in the New Testament (e.g., 1 Tim 2:5 [discussed below]), there is a distinction, not just between the persons of the Father and the Son, but also between “God” and the Son, which is very non-Trinitarian. This is the case in John 17:3. The Greek reads:
αὕτη δέ ἐστιν ἡ αἰώνιος ζωὴ ἵνα γινώσκωσιν σὲ τὸν μόνον ἀληθινὸν θεὸν καὶ ὃν ἀπέστειλας Ἰησοῦν Χριστόν.
"Now this is life of the age to come that they may know you the only one who is the true God and the one whom you sent, Jesus Christ" (my translation).
The title, τον μονον αληθινον θεον (“the only one who is the true God”), is predicated upon a single person, not a “being” composed of three “persons” (however one wishes to define “person”), and such is predicated upon the singular person of the Father, with Jesus himself distinguishes himself in John 17:3 from “the only true God.” Absolutising this verse, this is a strictly Unitarian verse as only a singular person is within the category of being the “only true God.” However, in Latter-day Saint theology, “God” is a multivalent term, something Trinitarianism cannot allow when speaking of (true) divinities. That this is the Christological model of “Biblical Christianity” can be seen in many places, such as Heb 1:8-9:
But unto the Son he saith, Thy throne, O God, is forever and ever; a sceptre of righteousness is the sceptre of thy kingdom. Thou hast loved righteousness, and hated iniquity, therefore, God, even thy God, hath anointed thee with the oil of gladness, above thy fellows.
This is an important pericope for many reasons—this is one of only a few places in the New Testament where Jesus has the term "God" (Greek: θεος) predicated upon him (others would include John 20:28 and probably, based on grammar, Titus 2:13 and 2 Pet 1:1], and yet, post-ascension, Jesus is differentiated, not simply from the person of the Father (ambiguously tolerated in Trinitarianism), but a differentiation from God (literally, the God [ο θεος]), something not tolerated in Trinitarianism. This can be further seen in the fact that this is a "Midrash" of Psa 45:6-7, a royal coronation text for the Davidic King, of whom Jesus is the ultimate fulfillment (cf. 2 Sam 7). Both the Hebrew and the Greek LXX predicates "God" upon the king, and yet, there is a God (in the case of Jesus, God the Father) above him. The LXX reads the same as Hebrews; the Hebrew literally reads "elohim, your elohim" (alt. "God, your God" [ אֱלֹהִ֣ים אֱ֭לֹהֶיךָ (elohim eloheyka)].
LDS scholar, Blake Ostler, accurately summed up the LDS understanding of the Father being “the only true God” in the following comment:
The Mormon scriptures make clear that there is a "God of all other gods" (D&C 121:32). It is implicit in this scriptural assertion that, although there is more than one individual of the kind "God," there is only one who is preeminent and the God of all. Such language is similar to the New Testament language that the Father is "the God and Father of Jesus Christ" (2 Cor. 1:3), even though it calls Christ "God." The book of Abraham says that this God of all other gods "is more intelligent than they all" (3:19). In any event, it is clear that the "one God" of the Old Testament is identified with the God and Father of Jesus Christ in the New Testament (1 Cor. 8:6, 1 Tim. 2:5). The Father is "the God" (John 1:1, emphasis mine) and "the only true God" (John 17:3).Such expressions entail that the Father is in some sense unique and superlative. (Blake T. Ostler, Exploring Mormon Thought, vol. 3: Of God and Gods [Salt Lake City: Greg Kofford Books 2008], 257-58).
Realising the problematic nature of this verse to Trinitarian sensibilities, Augustine had to butcher the verse in tractate CV of his Tractates on the Gospel of St. John:
"And this," He adds, "is eternal life, that they may know Thee, the only true God, and Jesus Christ, whom Thou hast sent." The proper order of the words is, "That they may know Thee and Jesus Christ, whom Thou hast sent, as the only true God."
As some scribes did to 1 Tim 3:16, Trinitarian proponents are forced to corrupt the text to support their dogma—this is strong evidence of the doctrine being man-made, not based on sound exegesis of the biblical texts.