There are already many articles written on the subject of the Holy Trinity. I just want to share some clear evidence from Scripture to debunk Binitarianism specifically. Binitarianism is the belief that there is no Third Person in the God-head and the Holy Spirit mentioned in Scripture is either the spirit-being of God the Father or God the Son.
Firstly, 2 Cor 13:14 mentioned all three persons in the benediction in which there is no necessity to mention the Holy Spirit if it is not existent separately. ( 2 Cor 13:14. May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all.)
Secondly, during the baptism of our Lord Jesus, it was clearly mentioned that God the Father spoke from Heaven and the Holy Spirit descended like a dove and alighting on our Lord Jesus. (Matt 3:16-17. As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”)
Thirdly, our Lord Jesus mentioned that He had to leave in order for God the Father to send the Helper (the Holy Spirit) to the disciples. John 16:7. And the Holy Spirit will not speak on His own authority but will only speak what He hears and will glorify our Lord Jesus implying the subordination of the Holy Spirit to Him. So there is clearly this subordination to one another in the God-head. God the Father would send God the Spirit to be subservient in His ministry to glorify God the Son and will not speak on His own authority but declare the truths from God the Son. John 16:14. These verses – John 16:7, 13-14 – will be incomprehensible if God the Spirit is not a separate Person from God the Father and God the Son. According to Binitarianism, if the Holy Spirit is just the spirit-being of God the Father or God the Son, re-reading these verses will be incomprehensible. (John 16:7,13-14. Nevertheless, I tell you the truth: it is to your advantage that I go away, for if I do not go away, the Helper will not come to you. But if I go, I will send him to you…When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. He will glorify me, for he will take what is mine and declare to you.)
Fourthly, the Holy Spirit intercedes for us. If the Holy Spirit is the spirit-being of God the Father, then it will be ambiguous to intercede to Himself since we know from John 16 that our Lord Jesus is distinct from the Helper, the Holy Spirit. (Rom 8:26-27. Likewise the Spirit helps us in our weakness. For we do not know what to pray for as we ought, but the Spirit himself intercedes for us with groanings too deep for words. And he who searches hearts knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God.)
Lastly, we are given the Great Commission by our Lord to make disciples of all nations and baptizing them in the name of the Triune God – God the Father, the Son and the Holy Spirit. In Acts 19:1-7, the disciples in Ephesus did not even know of the Holy Spirit despite being baptized by John the Baptist and telling them to believe in the name of Jesus our Lord. They did not have the Holy Spirit since their understanding of the gospel and the Triune God were likely to be spurious.
Through the centuries since the birth of the church, creeds from ecumenical councils and confession of huge part of Christendom have affirmed the Holy Trinity.
The Nicene Creed, accepted by all Christendom, clearly states in the creed that the Holy Spirit is worshiped and glorified together with God the Father and God the Son : “…And we believe in the Holy Spirit, the Lord, the giver of life. He proceeds from the Father and the Son, and with the Father and the Son is worshiped and glorified…” The Athanasian creed affirms the Holy Trinity though it is not universally accepted by all Protestant churches. (http://www.prca.org/about/official-standards/creeds/ecumenical/athanasian).
Other confession through the centuries include the Belgic Confession (https://www.ligonier.org/learn/articles/belgic-confession-1561/) and the Cape Town Commitment (https://www.lausanne.org/content/ctcommitment#p1-5).