Why is the Trinity important to apologetics? Well, what happens when unitarianism (the view that God is merely one) is substituted for Trinitarianism? One result is that the God so defined tends to lose definition and the marks of personality. In the early centuries of the Christian era, the Gnostics, the Arians, and the Neoplatonists worshipped a non-Trinitarian God. That God was a pure oneness, with no plurality of any kind. But one what? A unity of what?…

Anti-Trinitarianism always has that effect. It leads to a “wholly other” God, rather than a God who is transcendent in the biblical sense. Paradoxically, at the same time, it leads to a God who is relative to the world, rather than the sovereign Lord of Scripture. It leads to a blank “One” rather than the absolute personality of the Bible. It makes the Creator-creature distinction a difference in degree rather than a difference of being.

John Frame, Apologetics to the Glory of God (P&R, 1994), 47-48.