For my PhD in theology, the doctrine of the Trinity was one we discussed and I wrote several papers on the topic, with over 100 references, including entire 1000 page books. So, not likely we will cover it all on this thread. And of course, in seminary, a place some people seem to despise, the Trinity is a major topic of theology. At least in my conservative seminary!
Suffice it to say, my study of Scripture verifies the Trinity in the Bible. I believe anyone who disagrees with the importance of the Trinity is using out of context verses to support their heresy. Not believing in the Trinity is a huge heresy, which the church has fought since the beginning, including believing that Jesus was not God, Arianism, (4th century AD); docetism, (2nd century AD) believing Jesus was not fully man or physical, even on earth; Sabellianism or believing God is three faces or modes rather than three distinct persons, or hypostatis as the Greek says. One being, ousia, and three persons - hypostatis.
According to Stanley Grenz, each trinitarian person fulfills a unique and specific role. In classical theology, this assertion is: The Father creates the world, through the Son, by the Spirit. Further, the dynamic that binds the Father and Son - the power of their relationship - is the Holy Spirit. In this sense, the Spirit is likewise the essence of God, namely love. This divine essence between the Father and Son lies behind the act of God creating the world. Because God is precisely this dynamic and this love, the Holy Spirit is the one through whom the Father, the direct agent of creation, fashions the world. In other words, the Spirit is the personal power of God - the dynamic of love between the Father and the Son - by means of which all things exist! And a person in his own right!
While the doctrine of the Trinity was formalized by the church during the Patristic era, it was not an aspect of the gospel proclaimed by the New Testament church. But, the church is right in recognizing that understanding God as triune is a non-negotiable dimension of the Bible. The concept of a tri-unity lies at the heart of the Christian understanding of God and therefore is necessary in order to maintain the central message of the Bible. While it is true that the doctrine of the Trinity is the product of a lengthy process arising from the experience of the people of faith, it does not in any way disparage the vital importance of the Trinity, not just in soteriology or salvation, but in every single doctrine of God, the church and humans. The doctrine of the Trinity is grounded simply and directly in the self-revelation of God in Jesus Christ.
Therefore, the doctrine of the Trinity, rather than being of secondary importance, it is at the heart of God. In fact, the doctrine of the Trinity, is a practical doctrine that shapes Christian life. The Trinity establishes the unifying doctrines of grace, Christology, soteriology, pneumatology and eschatology, and therefore the mysterious reality that lies at the very heart of Christian faith.