Below I've copied an e-mail I recently sent to an inquirer at our congregation who seeks to learn more about the faith, and has lots of good questions. On Sunday, she asked about the Trinity. Below I've copied my response. What did I miss? What would you say differently?
I thought I'd also share with you briefly why I as a Christian confess the Trinity. These are rather impromptu notes, but they probably are representative of my thoughts on the sbject.
First of all, I believe the doctrine of the Trinity is primarily something that helps us understand how Christ is both God and man. If it weren't for Christ, we would have no reason to concern ourselves with the Trinity. The Trinity is not an abstract concept, some kind of philosophical formula to try and define God. Rather, we believe in the Trinity because of Christ.
Why do we believe in the Trinity, and how does this spring from Christ? Well, a good starting answer is to read the gospel of John. In the gospel of John, we get Jesus praying and teaching about his relationship to the Father and to the Spirit. The idea of the Trinity is about relationship. Jesus is in relationship with the Father because the Father is the Father of the Son, Jesus. He teaches his followers to pray to the Father in this way, "Our Father, who art in heaven..." And he also promises that, after he is resurrected and ascends to heaven, he will send the Holy Spirit to "be our comforter" and "lead us into all truth". So, the Holy Spirit comes to continue preaching and teaching Jesus the Christ to us.
This means that the God we know is not some distant and abstract deity, a creation of the minds of the philosophers. Instead, God is the one we know because Jesus Christ has come in the flesh, born of the virgin Mary, and lived as fully God and fully man. Because of this, we see that God cares for us (he sent his only Son) and God is intrinsically in relationship (because rather than being a lonely and all-powerful figure, God is a relationship, and mutual living together of the Father, Son, and Holy Spirit). I find this last aspect of the Trinity especially compelling. Because God is Trinity, God is in relationship in God's very self, and therefore, God is love. God could not be love (as 1 John tells us) unless God were three persons, for love is that which is between persons. So, we can say that God is love, and God loves, because God is already the lover, loving his beloved (Christ) in the power of love (the Holy Spirit).
All of these persons, Father, Son, and Holy Spirit, share intimately in each other's lives. The Father brought about all creation through the Word, the Son. This Word was in the beginning with God (John 1), the Holy Spirit does nothing but preach this Word, and the Spirit is present in Christ, and descends upon him. The members of the Trinity are therefore full participants in each other's lives, while remaining differentiated. This is also how we understand the body of Christ, into which we are incorporated at baptism and the Eucharist. Although it is true that the church is "one body", we also know that the body has many members, and therefore many different gifts and functions. The Trinity is the same. All members of the Trinity are fully God, but they are at the same time unique. The Father is not the Spirit, etc.
I find this to be wonderfully compelling, for it makes God dynamic, it teaches us how to understand the life of Jesus in relation to God and the Spirit, and it gives us promises- that God is love, that Christ is for us, and the Spirit remains with us, teaching us about Christ and leading us to the Father. This is why we baptize in this name and only in this name- I baptize you in the name of the Father, and of the Son, and of the Holy Spirit. We baptize only in this name because there is so much about this name worth speaking about and remembering.