It is usually alleged by unitarian/unipersonal (i.e., groups that believe God exists as one sole person, thus denying the Trinity) that the OT is entirely unconscious to the idea that God is multi-personal—the one God. Remember, that the divine truth and the way in which God unfolded that truth in the history of redemption has been progressive. Thus, as it has been asserted that the OT is the NT concealed, whereas the NT is in fact, the OT revealed.
So while such truths as the incarnation, the substitutionary atonement of the Redeemer exist primarily in the shadows of OT narrative, poetry, prophecy and their fulfillment in the fuller revelation of the NT, is perfectly consistent in the singular theme of God’s purpose among men: His work of salvation in Christ, to the praise and glory of His grace (cf. Heb. 1:1-2). The doctrine of the Trinity as defined in the fourth and fifth-century creeds is not contained in the OT, in terms of the specific doctrinal language. But, it does not follow to assert that because the OT utilizes different language than that of post-Nicene language, that this somehow militates against the notion that the Jews did, in fact, envisage God as multi-personal.
Monotheism & the Word “One”
As stated, groups such as Jews, Muslims, Jehovah’s Witnesses, and of course, Oneness believers are functionally defined as unitarian, for their commitment to absolute unipersonalism/unitarianism, which maintains the notion that God is unipersonal (i.e., one divine Person). This is, of course, why these groups flatly reject the doctrine of the Trinity—thinking that the Trinity is three separate Gods. However, in Hebrew there were various words that could be translated as “one” and the word that speaks of God being “one” (in the OT), every time, is echad (אחר, e.g., Deut. 6:4).
As many have pointed out, the term echad can indicate compound or composite unity—not necessarily absolute solitary oneness, as in Genesis 2:24, for example: “Adam and Eve became one [echad] flesh” (also see Gen. 11:6; 2 Chron. 30:12). Further, the word in the OT lingua franca, which does strictly signify absolute solitary oneness, is yachiyd (cf. Ps. 68:6), but this term is never once applied to God. If God were an absolute lone unipersonal Deity, as anti-Trinitarians assume, surely the biblical authors would have used the term yachiyd to say that God is “one,” but they did not, they exclusively used echad.
The Plurality of Persons Expressed
Aside from the first person plural verbs, adjectives, and prepositions used of God in the OT (i.e., “Let Us,” “Make Our,” “[One] of Us,” et al; Gen. 1:26-27; 3:22; 11:7-9; Isa. 6:8; 11:7; Isa. 6:8; 54:5 [lit. Heb., “Makers,” “Husbands”]; also “Maker” is plural in Ps. 149:2 and Job 35:10; Prov. 30:3 [qadoshim, lit., “Holy ones”; same with LXX–hagiōn]; Eccl. 12:1 [lit. Heb. “Creators”]) to underscore the multi-personality of God.
Similar with Jesus’ usage of first person plural verbs (eleusometha, “We will come,” and poiēsometha, “We will make”) to both Himself and His Father, in John 14:23 clearly distinguishing Himself from His Father). Note both Gen. 1:26 (LXX) and John 14:23 contain the same plural verb of poieō (“to make”). Thus, OT clearly presents Yahweh as multi-personal. Thus, because God is tri-personal He can be described as both “Maker” and “Makers” and as “Creator” and “Creators.” He is one Being, not one Person—a point that is repeatedly demonstrated by the OT authors.
The idea that God is an undifferentiated unipersonal Being is simply foreign to the OT message itself. Note some examples below: poieō
Yahweh to Yahweh
In Genesis 19:24, we read of the LORD’s wrath on Sodom and Gomorrah:
The sun had risen over the earth when Lot came to Zoar. Then the LORD [Yahweh] He rained on Sodom and Gomorrah brimstone and fire from the LORD [Yahweh] out of heaven (vv. 23-24).
Notice that it was the Yahweh, who rained brimstone and fire from the Yahweh out of heaven. Two distinct divine Persons called “Yahweh,” nothing more nothing less, if of course, you take Scripture on its own merit. But Unitarians cannot do so; their allegiance to their prior assumption that “God is unipersonal precludes even the possibility that such evidence might be considered objectively.
Psalm 45:6-7: Elohim to Elohim
Your throne, O God, is forever and ever; A scepter of uprightness is the scepter of Your kingdom. You have loved righteousness and hated wickedness; Therefore God, Your God, has anointed You With the oil of joy above Your fellows.
Not only do we have a clear multi-personal reference of Elohim (“God”) speaking to Elohim in direct address, but, the writer to the Hebrews applies this very text to the “Son” (not the Father), who Oneness teachers say is not God—only the Father is God.
The author of Hebrews quotes the Father directly addressing the “Son” as ho Theos, “the God.” For God (the Father) speaking to God (the Son) is consistent with the doctrine of the Trinity; two divine Persons differentiated from one another, yet each equally identified as “God.” God the Father speaking to God the Son.
The Angel of the LORD
We also see clear multi-personal references of God as we read of “the angel of the LORD [Yahweh]”. This angel was not some indefinite angel, one among many. This angel, who was called “the angel of the LORD,” claimed that He was “the God of Abraham, the God of Isaac, and the God of Jacob” (Exod. 3:6). When Hager encountered the angel of the LORD (cf. Gen. 16:7ff.) being frightfully responsive (due to of Exod. 20:19: “for no man can see Me and live”), said to Him, “You are a God who sees . . . Have I even remained alive here after seeing Him?” (16:13). There are many “angel of the LORD” references (e.g., Gen. 22:9-14; Exod. 23:20-21; Num. 22:21-35; Judg. 2:1-5; 6:11-22; etc.). Most significant is the account recorded in Judges 13:1-25 where Manoah and his wife (Samson’s parents) dialogued with this angel. And when Manoah discovered that it was “the angel of the LORD” he declared to his wife, “We will surly die, for we have seen God” (v. 22).
The “angel of the LORD” was clearly identified as Yahweh to those who interacted with Him. Yet in Zechariah 1:12, however, we find the angel of the LORD (who claimed to be Yahweh throughout the OT) praying to the “LORD [Yahweh] of hosts,”—Yahweh praying to Yahweh.
All these OT plural descriptions of the one Yahweh can only be consistent with monotheism in the context of Trinitarianism.
The biblical doctrine of the Trinity is arguably the pinnacle of God’s self-disclosure to mankind. From the multi-personal references of God in the OT to the personal distinctions between Father, Son, and Holy Spirit expressed in the NT (cf. Matt. 28:19; 2 Cor. 13:14), the triune nature of God has been well established. Despite this evidence, however, it seems that the preaching and teaching of the truth of the Trinity is largely absent from many Christian pulpits. Moreover, though some notable scholars have produced worthy contributions on the subject there appears to be a definite lack of ecclesiastical material, apologetic literature and other resources affirming and defending the doctrine of the Trinity.
The biblical conclusion: God in three Persons
When we proclaim the gospel of Jesus Christ we must proclaim the truth of God: God the Father, God the Son, and God the Holy Spirit. When the person of the Son is detached from the Trinity, the very Being of God is confounded. To deny the Trinity denies the person of the Son, and thus, the very essence of God:
22 “Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. 23 Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also” (1 John 2:22-23).
 E.g., Jehovah’s Witnesses, Oneness Pentecostals, Jews, Muslims, etc. etc.