The point of Trinitarian theology is not simply that there are three instead of one. We can’t just plug our pre-conceptions about God into the Trinity, and say that now we are Trinitarian. We can’t assume Deism, the idea that God is a watchmaker who leaves the world to run on its own, and then say that we are Trinitarian because we believe in three watchmakers. We can’t assume that God is sheer power like Allah, and then say we are Trinitarian because we believe in three Allahs and not just one.
To confess the Trinity is not merely to confess a number, but to confess that the true God is a certain kind of God. What kind of God?
For Paul, the answer to that question has everything to do with Jesus.
Read more at http://www.firstthings.com/blogs/leithart/2015/12/confessing-trinity