Pieper, Francis. Christian Dogmatics: Volume 1. St. Louis: Concordia Publishing House, 1968.
Chapter C5, “The Doctrine of the Trinity in the Old Testament”
Many through history have questioned whether the trinity is present in the Old Testament. Pieper affirms that the trinity is present, though not presented as clearly as in the New Testament. The argument takes a few steps, but it is present.
First, the Apostles and Jesus demonstrate the doctrine of the son and the Spirit using the Old Testament (Matthew 22:41ff, cf. Ps. 110:1). We especially note Hebrews 1, which demonstrates the divine person of the Messiah by using six Old Testament passages. The Messiah is clearly a divine person. The Old Testament is also clear that the Holy Spirit has a divine personality. Pieper cites Genesis 1:2; 6:3; 2 Samuel 23:1-3; and Isaiah 63:10 in particular.
Because the Son and Spirit are identified as divine persons, along with the Father, it is natural to think of the Trinity when God speaks of himself in the plural or where there is a threefold expression of worship or prayer.
Pieper does a good deal of biblical exegesis in this chapter as he surveys the idea in the Old Testament.
Chapter C6, “The Trinity and Human Reason”
Each person of the Trinity is a full and complete expression of the Godhead. This is, in fact, a logical impossibility. Yet it is precisely what the Bible teaches. Pieper details two ways that we attempt to explain the Trinity. First, though there are many natural analogies of three in one, all of them break down. Nature certainly bears God’s fingerprints, but never his unique and trinitarian nature. Second, we cannot adequately analyze the doctrine by considering special characteristics of the persons. This always results in some god other than the one of the Bible. We are left, then, with the biblical narrative and nothing else.
Pieper divides modern theology and its speculations into two categories. First, some will fall into unitarianism, depicting one God but three different powers or wills. Another group will so classify the persons of the Trinity that they become tritheists.
In the end we have a biblical account which cannot be adequately analyzed or understood by our reason. Yet, trusting that God has inspired his Word and has shown himself trustworthy, we accept and receive this Word.