Sometimes people ask why they should believe in the Trinity over a different monotheistic God. Besides the fact that I think this is how the true God has revealed Himself through history (including through the writing of His inspired word), the Trinity makes the most sense of the fact that we are relational beings.
It would be odd to think that we alone (who were created in God’s image), and not God Himself, were by nature relational beings who interacted with and loved others. If God were one lone Person, not three Persons in one being, in what sense could we say God is love? Jared Wilson comments on this in his book Unparalleled: How Christianity’s Uniqueness Makes It Compelling:
Love is not God. But God is love. So what does it mean for God to be love?
It does not necessarily mean that God is simply loving. Judaism and Islam and Mormonism teach a God who loves. But when Christians teach that God is himself love, they are saying that real love itself has its origin and its essence in God. And this cannot be true unless God is a Trinity.
Think about it: A solitary god cannot be love. He may learn to love. He may yearn for love. But he cannot in himself be love, because love requires an object. Real love requires relationship. In the doctrine of the Trinity we finally see how love is part of the fabric of creation; it is essential to the eternal, need-nothing Creator. From eternity past, the Father and the Son and the Holy Spirit have been in community, in relationship. They have loved each other. That loving relationship is bound up in the very nature of God himself. If God were not a Trinity but merely a solitary divinity, he could neither be love nor be God!
If it weren’t the case that love is part of the Creator by nature, then that would mean love didn’t exist until He created beings outside of Himself; either He was complete without love—in which case, we’ve been mistaken about the value of love—or He was lacking something before He created other beings to love. I don’t think we’ve been mistaken about the value of love. A loveless god would be a deficient god, and love would not be part of the foundation of a universe created by that god. Yet relationships and the desire for love are central to who we are. This would be a very odd state of affairs in a universe not created by a Trinitarian God.