The Trinity is the doctrine that “God is one essence in three co-eternal and co-equal persons” (Rob Green, Lafayette). In the scriptures, we see each person of the Godhead working in distinct, unique, divine (and sometimes overlapping) capacities and yet the Bible declares there is only One God. We are not saying three Gods (tri-theism) or three forms (modalism). We are saying that the one God exists and always has existed in three distinct persons - yet remains one God.
Let’s start with the biblical assertion that there is only one God (“one essence”). It is plain in Exodus 20:1-3, "I am the Lord your God, and you shall have no other gods before me." This assertion is picked up in Deut. 32:39, "There is no god besides Me" and Isaiah 45:5, "Besides Me, there is no God." In each of these, God affirms the greatest reality in the universe: There is one God, and apart from Him there is no other.
He also shows Himself in three persons. Notice several hints of plurality in Genesis. In 1:26-28, God says, “Let us make man in our image, according to our likeness....” The word used to render ‘God’ is a plural word (Elohim, Holman Dictionary), and He conspicuously uses plural pronouns, “US/OUR/OUR.” Another example is Genesis 16:13 when Hagar sees and speaks with the angel of the Lord. She calls Him, El Roi, “the God who sees” and remarks, “have I stayed alive after seeing Him?” Similarly, in Genesis 32:24, ff., Jacob wrestles all night “with a man” who names him Israel because he “wrestled with God and man and prevailed.” Jacob named the place Peniel in verse 30, because he said, “here I saw God face to face and was preserved.” Both instinctively know that no one can “see his face and live” (later declared in Exodus 33:20), yet both refer to the One they saw as God. Still another example is Genesis 18:1, ff., when “the Lord” appears to Abraham. He says, “I must not hide from him what I am about to do to Sodom,” (18:17) and “I chose him” in 18:19, things only God could say. Abraham calls Him the “judge of the earth” in 18:25, and 19:29 says, “God destroyed the cities of the valley,” statements that can only refer to God. But in each of these three cases, they see God ‘in the form of a man’ and yet live. How was that possible?
Before answering that question, let’s take a look at the New Testament. At the start of the NT, Jesus Christ is revealed as “Immanuel - God with us” Matthew 1:23. This “God with us” is seen in many ways, including His displays of power over nature (“wind and waves obey Him” Luke 8:22-24), demonic forces (“If you are going to cast us out, send us into the herd of swine” Matthew 8:28-34), sickness (“healed many who were ill with various diseases” Mk. 1:34) and death (raised Lazarus from the dead John 11:1-45).
It can also be seen by multiple ‘theological’ declarations (Colossians 2:9 “in Him dwelt the fullness of the Godhead in bodily form”, “He was the exact representation of His nature” Hebrews 1:3, “He did not consider equality with God a thing to be grasped” Philippians 2:6) as well as by statements that He created all things (“Apart from Him was nothing made that was made” John 1:1-5; “by Him all things were made” Colossians 1:16). In John 1:1-5, it even says, “In the beginning was the Word, and the Word was with God, and the Word was God.” This “Word” is present “in the beginning” (remember Genesis 1:1?), and is both distinct from God (“with God”) and is God (“was God”). Like seeing God and living, how is that possible?
Similarly, we have clear evidence that the Holy Spirit (present at creation - “hovering over the face of the waters,” Genesis 1:2) is both a person and God. One indication of this is the Great Commission of Matthew 28:18-20 when Jesus enjoins us to baptize believers in the name of the Father, Son, and Holy Spirit. To include each of these persons as co-equal in baptism is a clear inference of deity and separateness. Another is the presence of the Spirit along with the Father at the baptism of the Son (Matthew 4). Another is Romans 8:26-27 when we are assured that the Holy Spirit intercedes for us with the Father with groans too deep for words and that the Father knows what the Spirit is thinking. Still another is Acts 5 when Ananias and Saphira lie about the amount of money they received for their property. Peter says in verse 3, “You have lied to the Holy Spirit” and in verse 4, “You have not lied to men but to God.” And finally, 1 Cor. 2:11-12 says, No one knows the thoughts of a man except the spirit of a man; even so, no one knows the thoughts of God except the Spirit of God. The implication here is quite clear: The Spirit is God.
Only one option explains why the Old Testament shows examples of ‘people seeing God and living,’ why the New Testament shows ‘Jesus being in the beginning with God and being God,’ and ‘lying to the Holy Spirit is lying to God.' Namely, God exists as three persons: the Father – who cannot be seen, the Son – who has been seen (and who was seen in pre-incarnate form as the angel of the Lord in the Genesis passages we referred to earlier), and the Holy Spirit who points men to the Father through the Son.