Many people seem to assume that the God revealed in the Old Testament is ‘God the Father,’ awaiting the unveiling of God the Son and God the Holy Spirit in the New Testament, and so completing the revelation of God as Holy Trinity. This is a profoundly mistaken view of the Bible. Even the bare fact that John 12:38 sees Jesus as the fulfillment of Isaiah 53:1 should warn us against making the Old Testament the province of only God the Father! No, no, the God of the Old Testament is the ‘Lord of hosts,’ the God whose unity is a rich diversity, as yet unfocused and undefined. But as soon as we enter the New Testament, as by a simple adjustment of the focusing lens, we are allowed to be present when the voice of the Father addresses His Son, when Jesus is identified as the Son of God, and when the Holy Spirit descends, bodily as a dove, to rest on Jesus and remain with Him. And in that incomparably beautiful and simple way, the ‘hosts’ are brought into their correct and final focus, the ‘incognito’ is over, and the Holy Trinity is at last revealed.
— J Alec Motyer, A Christian’s Pocket Guide to Loving the Old Testament