This question comes from Mario:
"How does one defend the trinity? And does it matter? I think of Mt 28:19 But it's not so much of a formulated three-in-one. I think of John 1 God is the Father. Jesus is the incarnation of God's self-expression (or verb) It seems more of a traditional creed. I think if I grabbed Paul and talked to him about a trinity, he'd look at me with confusion."
To begin, I do think that the doctrine of the trinity is an important one, and I believe it is defensible, as it is progressively revealed in Scripture (meaning there are roots in the Old Testament, being more fully revealed in the New). While the doctrine of the trinity is a difficult one to understand, much less explain, I don't think that makes it unimportant or irrelevant.
You are correct that it seems it wasn't a formalized doctrine until after the early church (c.f. Nicene Creed 325 A.D.), but I think the fact that it became a formalized doctrine can give us insight into it's importance. I'm not sure about your comment about what Paul's reaction would be to the concept of the Trinity, as it is of course speculation, but I'm not so sure that he would be confused. It seems that Paul certainly saw Jesus as the Son of God and the Holy Spirit as the Spirit of God, so perhaps his vernacular would be different than ours, but the concept of these different aspects of God, yet being God, are congruent within his writings and the other writings in the New Testament (c.f. Jn 1:1-4, 20:30-3; 1 Cr 12:4-6; 2 Cr 13:14; Eph 4:4-6; 1 Pet 1:2; Jude 20-21).
I hope this helps, for a more thorough discussion on this topic you can see the chapter on the Trinity in Systematic Theology by Wayne Grudem (pg. 226-261).