Let’s consider two more reasons for the critical importance of the historic Christian doctrine of the Trinity.
First, the Trinity doctrine sets the Christian view of God apart from all other religious perspectives, including other monotheistic viewpoints (such as Judaism and Islam). Only the Christian God is one in essence but three in personhood (in philosophical terms: one What and three Whos). This triune nature means that God is superpersonal (more than merely a personal being).
Trinitarian monotheism also solves philosophical problems for which unitarian concepts of God (God as a single solitary person) have no answer. For example, the greatest of the Christian church fathers, St. Augustine (AD 354-430), explained in his monumental work On the Trinity that only a God who has plurality within unity can be both loving and knowing. For if God is a single solitary person, then before the creation he had no one to love; nor could he have distinguished between the knower and the known (a requisite of self-knowledge). In this way, the Trinity becomes quite practical. Because human beings are created in the image of the fully relational triune God, concepts such as love, family, and community take on new meaning and value. Redemption in Christ is properly understood as adoption into the family of God.
The doctrine of the Trinity reveals God’s very nature and personhood and sets the faith apart from all other religions. It also reveals God as an eternally relational being. Today’s believers join with believers of centuries past in worshiping “one God in trinity and the trinity in unity.” The Christian Gospel of redemption is from first to last directly connected to the triune God. As theologian Bruce Milne notes: “Just about everything that matters in Christianity hangs on the truth of God’s three-in-oneness.”
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.