As discussed, the absence of the article before the first personal noun in the Pauline salutations, clearly distinguishes the Persons of the Father and the Son. Even more, the real personal distinction between the Persons of the Trinity is well observed in constructions where the article is repeated before all of the personal nouns.1 For example, Reformed theologian B. B. Warfield shows that the repeated article in Matthew 28:19 demonstrate that the three Persons are numerically distinct and are under the one Name: Jehovah:
He commands them to baptize their converts ‘in the name of the Father and of the Son and of the Holy Ghost.’ The precise form of the formula must be carefully observed. It does not read: ‘In the names’ (plural)—as if there were three beings enumerated, each with its distinguishing name. Nor yet: ‘In the name of the Father, Son, and Holy Ghost,’ as if there were one person, going by a threefold name. It reads: ‘In the name [singular] of the Father, and of the [article repeated] Son, and of the [article repeated] Holy Ghost,’ carefully distinguishing three persons, though uniting them all under one name. The name of God was to the Jews Jehovah, and to name the name of the Jehovah upon them was to make them His. . . . (Warfield’s brackets).2
A “repeated article,” Harris explains, “shows unambiguously that nouns are separate items.”3 As well, Paul’s grammar clearly denotes a presentation of three distinct Persons—not three modes of a unipersonal deity:
The grace of the Lord Jesus Christ [tou kuriou Iēsou Christou], and [kai] the love of God [tou theou], and [kai] the fellowship of the Holy Spirit [tou hagiou pneumatos] be with you all (2 Cor. 13:14).
In Revelation 5:13, the Father and the Lamb are presented as two distinct objects of divine worship, differentiated by the repetition of the article tō:
“To Him [tō] who sits on the throne, and to the Lamb [kai tō arniō], be blessing and honor and glory and dominion for ever and ever” (emphasis added).
There are many other passages where this “construction of distinction” (viz. Sharp’s rule #6) applies, clearly demonstrating the real distinction between the three Persons of the holy Trinity (e.g., Matt. 28:19; 1 Thess. 3:1; 2 Thess. 2:16-17; 1 John 1:3; 2:22-23).
Subject-Object Distinctions: Simply, if Jesus and the Father were not distinct cognizant Persons, we would not expect to find a clear subject-object relationship between them:
After being baptized, Jesus came up immediately from the water . . . behold, a voice out of the heavens said, “This is My [subject] beloved Son, [object] in whom I [subject] am well-pleased” (Matt. 3:16-17; emphasis added; see also 17:5).
“I [subject] glorified You [object] on earth, having accomplished the work which You [object] have given Me [subject] to do” (John 17:4; see also Luke 23:34, 46).
The Father and the Son stand in an “I”–“You” relationship of each other; the Son refers to the Father as “You” and Himself as “I.” The Father likewise refers to Jesus as “You” and Himself as “I.” The Son personally relates to the Father and the Holy Spirit, and the reverse is altogether true of the Father and the Holy Spirit relating to each other.
Salvation is predicated inextricably on the Tri-Unity of God. Scripture knows of no other God. God the Father infallibly saves according to His mercy whereby the sinner is regenerated by means of the Holy Spirit, through the God the Son, Jesus Christ:
He [God the Father] saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit, whom He poured out upon us richly through Jesus Christ our Savior (Titus 3:5-6).
Also, as pointed out in several places, the OT uses plural verbs, nouns, adjectives, and prepositions to describe the one Being of God.
http://www.christiandefense.org/article_Trinity in the OT.htm
Notes
1 As seen above, in constructions involving multiple personal nouns linked by kai and the first noun lacks the article; each noun must denote a distinct person (viz. Sharp’s rule #5). However, when multiple personal nouns in a clause are each preceded by the article ho and linked by kai, each personal noun also denotes a distinct person (e.g., 2 Cor. 13:14; 1 Thess. 3:11; 1 John 1:3; 2:22-23; Rev. 5:13). This rule is known as Sharp’s rule #6 (cf. n. 39 above; also cf. Sharp, Remarks on the Uses of the Definite Article). Pertaining to Sharp’s rule #6, I had consulted Professor Daniel B. Wallace as to the question of its absoluteness and its utilization against the anti-distinction of the Modalists. He expertly affirmed that predominately (not absolutely; cf. John 20:28) the rule does provide a valid case against Modalism. Clearly, this rule indicates, particularly in Trinitarian contexts where all three Persons are juxtaposed in the same verse, a personal distinction between the three Persons of the Godhead.
2 Warfield, Biblical Doctrines, 204. Pertaining to the same passage (Matt. 28:19), Warfield further explained that
With stately impressiveness it asserts the unity of the three by combining them all within the bounds of the single Name; and then throws up into emphasis the distinctness of each by introducing them in turn with the repeated article: “In the name of the Father, and of the Son, and of the Holy Ghost” (Authorized Version). These three, the Father, and the Son, and the Holy Ghost, each stand in some clear sense over against the others in distinct personality: these three, the Father, and the Son, and the Holy Ghost, all unite in some profound sense in the common participation of the one Name” (ibid., 153-54).
3 Harris, Jesus as God, 310.
Salvation is predicated inextricably on the Tri-Unity of God. Scripture knows of no other God. God the Father infallibly saves according to His mercy whereby the sinner is regenerated by means of the Holy Spirit, through the God the Son, Jesus Christ:
He [God the Father] saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit, whom He poured out upon us richly through Jesus Christ our Savior (Titus 3:5-6).
As seen above in Revelation 5:13 the “Lamb” and the “Father” are presented as two distinct objects of divine worship:
“To Him [tō] who sits on the throne, and to the Lamb [kai tō arniō], be blessing and honor and glory and dominion for ever and ever” (emphasis added).
And the four living creatures kept saying, “Amen,”
And the elders fell down and worshiped.
*Note: “To Him” (the Father) and “the Lamb” (the Son) are grammatically differentiated by the repeated article, tō, (“the”) in which precedes both nouns and are connected by the one conjunction, kai, “and” (cf. Matt. 28:19; 2 Cor. 13:14; 1 John 1:3; cf. Sharp’s rule #6).
Distinction was indisputably in the Apostle John’s mind as he distinguishes the Father and the Son (“the Lamb”). Further, in 1 John 1:3, John shows that believers have fellow-ship with BOTH the Father and the Son. The Apostle John repeats the Greek preposition meta to show that the Father and Son are distinct from one another:
we proclaim to you also, so that you too may have fellowship with [meta, meta] us; and indeed our fellowship is with [meta, meta] the Father and with [meta, meta] His Son Jesus Christ (1 John 1:3).
In his Trinitarian benediction, Paul grammatically emphasizes the distinction (viz. by the repetition of the article tou, “the”) between the three divine Persons:
The grace of the Lord Jesus Christ [Iēsou Christou], and [kai] the love of God [tou theou], and [kai, kai] the fellowship of the Holy Spirit [tou hagiou pneumatos] be with you all (2 Cor. 13:14; cf. Matt. 28:19).