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.