Why the Triune God?

Why the Triune God? Why not just talk about God? Shouldn't we get God down first (a 'general' doctrine of God), and then move on to this "trinity" idea? This is a decision upon which everything stands or falls. Weber puts it like this:

In our doctrine of God's being and attributes, everything then will depend upon our really talking about the God who encounters us in the biblical witness. This God, however, is the One who discloses himself to us as the Father in the Son through the Holy Spirit. He is the God whose unity, life, and revelation are expressed by the Doctrine of the Trinity in reflection and interpretation.

Weber, 350.

What the point is, is that if there is some God behind the trinity (i.e., behind God as he has revealed himself in scripture), then we actually have no knowledge of God because God had actually not revealed himself. He would have revealed some other version of himself. If this were true, then all would be lost. All theology/talk of God would be utterly meaningless and nothing more than human speculation. But this is not the case. God reveals himself. He does so as he is, Father, Son, and Spirit. This is good news.