The first hint of the plurality of the unity of the Godhead is found early in the Bible. “Then God said, ‘Let Us make man in Our image, according to Our likeness’” (Genesis 1:26). The plural “Us” and “Our” is best understood in the light of the NT. This plurality is understood to be Three Persons in one. Trinity (threeness) is the common name, but Triune God is a better term for the Three in One. Neither word appears in the Bible. The complete revelation of the Triunity is given in God’s works rather than a statement, such as “God is Three Persons in One” or “Triune God.”
The Triunity of God is more clearly expressed in the NT and implied in the OT, for example, with the threefold ascription of praise by the Seraphim.
In the year that King Uzziah died, I saw the Lord seated on a high and lofty throne, and His robe filled the temple. Seraphim were standing above Him; each one had six wings: with two he covered his face, with two he covered his feet, and with two he flew. And one called to another: Holy, holy, holy is the LORD of Hosts; His glory fills the whole earth (Isaiah 6:-33).
In a probable messianic prediction, the Trinity is present.
“I have spoken; yes, I have called him; I have brought him, and he will succeed in his mission. Approach Me and listen to this. From the beginning I have not spoken in secret; from the time anything existed, I was there.” And now the Lord GOD has sent me and His Spirit (Isaiah 48:15-16).
The statement “has sent me and His Spirit” is either a reference to Isaiah or the Servant of the LORD. The NIV ends the Speaker’s announcement before this statement with a quotation mark.
Numerous conservative commentators have argued that the new speaker introduced here is in fact God’s Servant, the Servant of the songs. He is clearly not simply the Lord, and the phrase “Sovereign LORD” recurs in 50:4-5, 7, 9, in the context of the third Servant Song. Unlike the first and fourth songs, where it is God who speaks about him, here, as in the second and third, he himself speaks. Perhaps he is introduced here because Cyrus’s work is simply a harbinger of the much greater deliverance he would bring to God’s people (Zondervan NIV Bible Commentary).
Some see the Triunity in the threefold repetition of the name of the LORD in the Aaronic Blessing.
The LORD bless you and protect you;
the LORD make His face shine on you, and be gracious to you;
the LORD look with favor on you and give you peace.
In this way they will put My name on the Israelites, and I will bless them (Numbers 6:24-27).
It is not likely Aaron, the High Priest, envisioned the Triunity of the Godhead; however, the Holy Spirit, who inspired this blessing, could have desired this concept contained in it.
Whatever the case with the preceding passages, the NT takes up the implications of the OT, making the Triunity clear and plain.
Undoubtedly, God is the most complex being that exists. He is not simplistic and neither are concepts that explain Him. Reality is odd. It is not neat, not obvious, not what you expect. Reality in fact, is usually something you would have never guessed.
Christians believe what Jews believe: God is One. Because Christians also believe in the Triune God, they have been inaccurately accused of believing in three gods, a form of polytheism. Christians see a polarity within the Godhead, but that is not polytheism.
There is one body and one Spirit, just as you were called to one hope at your calling; one Lord, one faith, one baptism, one God and Father of all, who is above all and through all and in all (Ephesians 4:4-6).
The oneness of the Spirit with the Son (the Lord) and God the Father is apparent in this passage. The oneness of God is stressed in the Shema (Deuteronomy 6:4), which is repeated by Jesus in Mark 12:29, “Hear, O Israel: The LORD our God, the LORD is one.” There is only one God, not many!
So that from the rising of the sun to the place of its setting men may know there is none besides me. I am the LORD, and there is no other (Isaiah 45:6 NIV).
The word One (dxa ‘echad) in Hebrew can denote the cardinal (one), but often it denotes to be united with as one as in Genesis 2:24: “For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.” Deuteronomy 6:4 refers to one in isolation, while allowing for one in unity and diversity within unity. God is one God but also existent in three
“persons.” Person is one with the marks of personality described above. We remain a person when we die and leave our body. Our body does not make us a person in the sense used here.
God is not one and three in the same sense. God should be viewed as:
One as to His being
Three as to His personality
The Triunity must not be explained as three modes of existence, which is one God manifesting Himself in three ways. The Triunity is essential to the being of God and is more than a form of divine revelation. There are not three separate gods, like three separate human beings, such as Peter, James and John. The NT writers were thoroughly “Trinitarian” in their theology. They saw no incongruity between their doctrine of God and the monotheism of the OT.