In the New Testament, there are two books that reveal that God is Triune more clearly. In the first book of the New Testament, Matthew, we see how God became incarnated, born of a virgin, to be a man called Jesus (Matt. 1:21-23). The very God through the incarnation became a man. Then, after He died and resurrected, He became the Spirit (1 Cor. 15:45). This is why in the last chapter of Matthew, the Lord told His disciples to go and disciple the nations, baptizing them into the name of the Father, the Son, and the Holy Spirit (Matt. 28:19).
It is at this point in the Bible that the Triune God is clearly revealed. Now we know that the “Us” in Genesis 1:26 is revealed in Matthew 28:19 as the Father, the Son, and the Spirit. Does this mean there are three gods? No, there is only one God, because Matthew 28:19 speaks of three persons, but only one name. The Lord told them to baptize the nations into the name, not the names. Strictly speaking, the word “persons” is not adequate to describe the Triune God. If you press the word “persons” too far, you will have three gods. The Father, Son, and Spirit are one God revealed in the Bible.
Why must God be the Father, the Son, and the Spirit? As we have seen in the Old Testament, it is for God to deal with man. In the New Testament it is further revealed that it is for God to dispense Himself into us. The Triune God is for the dispensing of Himself into man.
We should not think that the Son of God is separate from God Himself. The Son of God is God expressing Himself (John 1:1, 14, 18). The Son of God is the expression of God Himself. And God is the Spirit to reach us and get into us. This is our Triune God. In the Gospel of Matthew, the Trinity is clearly revealed not as a doctrine, but as a dispensation. He is the Father, the Son, and the Spirit. Now He charges us to baptize people into the name of the Father, and the Son, and the Holy Spirit, that is, to baptize them into this Triune God. This is for dispensing Himself into us.
In the Gospel of John, it is the same. John does not mention the Father, the Son, and the Spirit all in one sentence as Matthew does. But John does mention the Father, the Son, and the Spirit many times. In this Gospel, the Lord Jesus speaks of how He was sent by the Father, and that He can do nothing of Himself: “I can of mine own self do nothing:...I seek not mine own will, but the will of the Father which hath sent me” (John 5:30).
The Lord Jesus also mentions in the Gospel of John that when the Son speaks, the Father does the works: “Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works” (John 14:10). How could one speak and another do the works? But the Lord Jesus says that He speaks and the Father does the works, because He and the Father are one. John 14:10 is a big verse! Here the Lord says that He is in the Father, and the Father is in Him. The words that He speaks to us are not of Himself, but the Father that dwelleth in Him does the work. In brief, the Lord is saying that He speaks and the Father does the works, because He and the Father are one.
Then in the same chapter, the Spirit is also mentioned. “And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; even the Spirit of reality; whom the world cannot receive, because it seeth him not. neither knoweth him: but ye know him: for he dwelleth with you, and shall be in you” (John 14:16-17, Gk.). So we see that John tells us about the Father, the Son, and the Spirit, and yet they are One. The Father is in the Son, and the Son becomes the Spirit (1 Cor. 15:45) in order to be in us forever. This is why the Lord prays in John 17, “That they all may be one; as thou, Father, art in me, and I in thee, that they also may be in us” (John 17:21).