I am going to provide some additional information to help in your understanding of this Biblical doctrine. This also will equip you to answer the Jehovah Witness who asks, “If the Trinity is such an important doctrine, why isn’t it taught in the Old Testament?”
The Doctrine of the Trinity is progressively revealed
As we have seen the New Testament clearly teaches there is 1 God that subsists in 3 persons Father, Son, and Holy Spirit. Some critics point out the doctrine of the Trinity can only be found in the New Testament. Even if this is true the doctrine is still Biblical. However, we will see how a case for the Trinity can be made from the Old Testament.
We need to realize that God most likely chose not to fully reveal his Triune nature in the Old Testament due to the fact that one of the major problems for the nation of Israel was fighting off polytheism. Polytheism is the belief in more than one true God; believing in multiple false gods. Throughout the Old Testament we see God reviling the nation of Israel for their worship of false gods. These Israelites would not have been ready to wrestle with one God subsisting in three persons. Especially, since the 2nd person of the Trinity had yet to take on a human body. It was only after God manifested Himself in human flesh as Jesus and the work of the Holy Spirit from Pentecost on, that we see the Trinity fully revealed in the New Testament. This demonstrates the progressive nature of Biblical revelation. When the time was right, more information about the nature of God was revealed.
However, if God has eternally existed in three persons, it would seem surprising not see indications of the Trinity in the Old Testament. Even though the doctrine of the Trinity is not explicitly taught in the Old Testament, several passages imply that God eternally existed in more than one person.
I will begin with Genesis 1:26, 27 “Then God said, ‘Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth.’ And God created man in His own image, in the image of God He created him; male and female He created them.”
In verse 26, it reads, “Then God said, ‘Let Us make man in Our image, according to Our likeness.’” Who is the “Us” or “Our” that God is talking about? Who is equal to God, so that we are created in their image? Additional verses that hint at this apparent plurality within the one God are Genesis 3:22 and 11:7. Genesis 1:27 helps solve this problem as it states, “God created man in His (singular) own image, in the image of God He (singular) created him.” How can God talk about Himself as plural verse 26 and in the next verse singular? The doctrine of the Trinity, one God subsists in three persons helps us best understand these verses. The “us and our” implicitly refer to the Father, Son, and Holy Spirit creating us in the image of God.
Another interesting verse is found in Psalm 110:1 (A Psalm of David) “The Lord says to my Lord: ‘Sit at My right hand, Until I make Thine enemies a footstool for Thy feet.’”
Jesus is going to quote this verse to give his listeners understanding about who he is in Matthew 22:41-46. “Now while the Pharisees were gathered together, Jesus asked them a question: 42 ‘What do you think about the Christ, whose son is He?’ They *said to Him, ‘The son of David.’ 43 He *said to them, ‘Then how does David in the Spirit call Him ‘Lord,' saying, 44 ‘THE LORD SAID TO MY LORD, “SIT AT MY RIGHT HAND, UNTIL I PUT YOUR ENEMIES BENEATH YOUR FEET’? 45 ‘If David then calls Him ‘Lord,' how is He his son?’ 46 No one was able to answer Him a word, nor did anyone dare from that day on to ask Him another question.”
When Jesus quoted this Psalm in Matthew 22, He knew David was referring to two separate persons as “Lord”. Who is David’s Lord (my Lord) other than God Himself? Who could say “sit at my right hand” other than someone who is fully God? From a New Testament perspective this verse could be paraphrased, “God the Father said to God the Son, ‘Sit at my right hand.’” It would seem, that even without the help of the New Testament, David clearly understood the plurality of persons within the one God. Here In Matthew 22 Jesus clearly understood this and when He asked the Pharisees for an answer, how could David’s Lord be his son, no one was able to answer Him, nor did anyone dare from that day on to ask Him another question. They couldn’t answer him because these Pharisees knew Jesus was identifying himself as the second Lord in the Psalm 110:1 passage. And as such Jesus was calling himself God. Just as the religious Jews during the time of Jesus struggled answering this passage, unless the Jews today are willing to admit one God and plurality of persons, they have no good explanation for this Psalm or Genesis 1:26, 27.
 Grudem, Wayne, Systematic Theology, Zondervan Publishing House, 1994, p. 228