The distinctive Christian claim is that God is love. A Jew or a Muslim would say that God loves or he loves the world, and that’s true. We say the same thing, but the radical Christian difference is we say that God is love. In other words, love isn’t just something that God does. It’s who God is, and the claim that God is love is the ground for the Trinity.
Chesterton said, “A lot of people are uneasy with the doctrine of the Trinity. It seems like medieval mumbo jumbo, but everybody gets enthusiastic about the claim that God is love. In fact, those two claims are the same.” If you say the claim that God is love, then you have to mean that within the very being of God, there’s a play of a lover, a beloved, and the love that they share. Yes, God loves the world, that’s true, but more primordially, God is love, and therefore, we have to say there’s a play of one, two, and three within the one God of Israel.
Here’s the amazing thing: John, Peter and Paul were proclaiming the God of Israel who was the hero of Israel. They were proclaiming the Lord is God alone. No Christian wants to deny for a second the unity of God. We reflect it, by the way, in our creed when we say, “We believe in one God.” That’s our version of hero Israel, “the Lord your God is God alone,” but yet, this one God of Israel has now revealed himself to be a play of lover, beloved, and love. We know that because Jesus the Son, who was sent by the one He called father, is Himself God. Abraham, Isaiah and Jeremiah were sent just as human prophets. Jesus was sent, but yet He is God. Therefore, within God Himself, there is some play of sender, sent, and the love which the sent was sent. I think that’s the ground for the Christian claim about Trinity.