There are many hints in the Old Testament through which we can know that God is triune, but it is difficult to see in the Old Testament that the Triune God is the Father, the Son, and the Holy Spirit. It is not until the end of the first Gospel in the New Testament that we see the composition of the Divine Trinity (Matt. 28:19b). The composition of the Father, of the Son, and of the Holy Spirit was not clearly and completely unveiled until after Christ's resurrection. After His resurrection, and before His ascension, He came back to the disciples and charged them to disciple the nations, baptizing them, the new believers, into the name of the Father and of the Son and of the Holy Spirit. In the Acts the apostles baptized people into the name of Jesus Christ (8:16; 19:5). This means that Jesus Christ equals the Father, the Son, and the Spirit. Before the man Jesus became the life-giving Spirit, the Divine Trinity was not fully consummated.
The Second of the Divine Trinity is the Son. He is the only begotten Son of God (John 3:16) and the firstborn Son of God (Rom. 8:29). Before Christ was incarnated, He did not have humanity; before His incarnation the Son was only divine. Furthermore, before His resurrection the Son was God's only begotten Son, not the Firstborn. In this sense, the Second of the Divine Trinity was not fully consummated before His resurrection. He needed to pick up humanity through incarnation, and He needed to become the firstborn Son of God through resurrection (Acts 13:33). So after His incarnation and resurrection, the Second of the Trinity was completed, consummated.
Now we need to consider the Third of the Divine Trinity— the Spirit. Before the incarnation and resurrection, the Spirit was only the Spirit of God, not the Spirit of Man. The Spirit of Jesus is the Spirit of Man. In the Spirit of God prior to the incarnation, there was no human living, no all-inclusive death, and no element of resurrection. In other words, before the incarnation and the resurrection, the Spirit of God was not compounded. It was through incarnation, human living, crucifixion, and resurrection that the Spirit of God was compounded with humanity and with Christ's death and resurrection. So after Christ's resurrection, the Third of the Divine Trinity was also consummated. After the resurrection, the Spirit of God is the life-giving Spirit, the Spirit of Jesus, the Spirit of Christ, the Spirit of Jesus Christ, and the Lord Spirit. All these aspects of the Spirit are for the consummation of the Triune God. The Triune God was consummated in Christ's resurrection, so after His resurrection the Lord came back to say that we are to baptize people into the name of the Father and of the Son and of the Holy Spirit.
The Triune God has been consummated in the life-giving Spirit, the Spirit of Jesus, the Spirit of Christ, the Spirit of Jesus Christ, and the Lord Spirit. So this Spirit today is the consummation of the Triune God.