Too many Christians, living as functional Unitarians, fail to recognize the Trinity’s relevance to their Christian faith and life. The Trinity doctrine is crucial because it reveals What and Who God is (one God in three persons), and this insight allows Christians, though in an obviously limited way, to view the inner working of God’s nature and personhood.
Furthermore, the Trinity doctrine brings together in a coherent manner the great truths about God’s historical, redemptive actions (completed in and through the Father, Son, and Holy Spirit). For example, (1) the Father (the first person of the Trinity) sends the Son into the world to offer a propitiatory sacrifice on the cross; that is, a sacrifice that both appeases the Father’s just wrath against sin and extends the Father’s love and mercy by allowing repentant sinners to escape divine judgment. (2) The Incarnate Son (the second person of the Trinity) is able to provide this atonement because he is both God and man. The God-man conquers death, sin, and hell through his glorious resurrection from the dead. (3) The Holy Spirit (the third person of the Trinity) is directly responsible for the repentant sinner’s new birth in Christ through regeneration, and the believer’s life journey of sanctification. The three divine members of the Trinity make the entire plan of redemption possible.
Redemption, therefore, in historic Christianity is initiated by the Father (Galatians 4:4), accomplished through the Son (1 Peter 3:18), and is applied by the Holy Spirit (Titus 3:5). The doctrine of the Trinity is important because there is no salvation apart from the Triune God. And the more we reflect upon God’s Triune nature, the more we can learn to love and appreciate God for Who and What he truly has revealed himself to be.
For more on the historic Christian doctrine of the Trinity, see “How Can God Be Three and One?” in my book Without a Doubt: Answering the 20 Toughest Faith Questions.