Is the Trinity revealed in the Old Testament? Is it revealed at all clearly in the Old Testament? Did the content of faith in the Old Testament include an awareness of distinct persons in the Godhead? Were Old Testament believers Unitarians with some dim perceptions of distinct divine persons?
One way of dealing with this issue is to claim that the Trinity could not have been revealed in the Old Testament because it would have hindered in some way the clear perception that God is One. This explanation is set against the backdrop of the rampant idolatry and polytheism of the Ancient Near East. Any clear disclosure of the Trinity would presumably have been distorted and corrupted by this theological context. But is this an altogether satisfying explanation?
We ought to remember that this is an inference that is being drawn from the presence of idolatry and polytheism in the Old Testament. As an inference, of course, this may well be acceptable. What I mean by that is simply that the creedal doctrine of the Trinity is itself an inference from the data of Scripture. But in the case of the denial of a clear disclosure of the Trinity in the Old Testament, is this a necessary inference? Can it be substantiated?
Perhaps one of the hidden assumptions of this inference is a devaluing of the content of faith in the Old Testament and even the capacity of Old Testament believers to conceive of plurality in the Godhead. This may be an acceptable working assumption for 19th Century liberalism and its offspring, but not for those whose anthropology is biblical. Are we to assume that conceiving of God in three persons was beyond them? Would Abraham and David have fallen headlong into polytheism if they knew, and believed, that the one true God existed as three distinct persons? Did they not scratch their heads when they met with the angel of God who is identified with God and yet somehow distinct from him? Should we say that this is an implicit Trinitarianism that needed to await the New Testament to be ratified as more than a theological puzzle? By calling it implicit what precisely do we mean?
A further concern with this inferential explanation is the unchanging presence and danger of idolatry and polytheism across the Testaments. Idolatry was just as pervasive in the world of the New Testament as it ever had been. What had really changed in the cultural setting by the time of the incarnation? And then of course we recognise that the apostles left their own record of warnings about the insidious danger of idolatry, warnings that we stand in need of today.
We ought also to remember that the doctrine of the Trinity was believed in the idolatrous polytheistic pagan world of the New Testament, and that largely by a people unschooled in centuries of exclusive monotheism. On the whole, although with the exceptions that Romans 11 points out, it was not monotheistic Jews that believed in the divine messiah but pagans. By joining themselves to the people of God they found that a new history was now theirs. But these Gentile converts had never been schooled in monotheism as Israel had been, and yet, as the pages of the New Testament confirm, they took up the doctrine of the Trinity in Unity with an unblinking certainty. Now if they could do it, how could we relegate the faith of the elect in the Old Testament, a priori, to have been incapable of holding to this high doctrine?
Doubtless a strong counter argument, held in wait mentally throughout the previous paragraphs, is to assert that the great redemptive facts of the New Testament, the incarnation of the Son and the outpouring of the Spirit, establish explicit faith in the Trinity to a level never found in the Old Testament. It is these redemptive facts and their revelatory significance that establish emphatically the doctrine of the Trinity.
And yet we do not have to search in vain in the Old Testament to find that Yahweh will send Yahweh (Zech. 2), and that God's omnipresent Spirit can be grieved (Isa. 63). Must we read these texts in such a way that excludes plurality within the Godhead on the assumption of cognitive incapability? Could it not be the case that the divine Messiah, the God-man, would come as promised? Could it not be the case that God's promised Spirit was conceived of in personal terms and not just as a personification of God's power? The great redemptive facts of the New Testament were first of all the great redemptive promises of the Old Testament. The theophanies of the Old Testament prepared the way for permanence of the incarnation.
Of course it could objected that this Old Testament evidence, even in prophetic form, is scattered and strewn across its pages. The doctrine is simply not presented in a coherent way in the Old Testament. Would that not be to confuse the presence of the doctrine with its systematic form? Is it not the case that the New Testament data of the Trinity also needs to be collated? Warfield spoke of the doctrine of the Trinity being revealed almost as a by-product of the way that God acted interpersonally in redemption. And from these great redemptive acts, interpreted and explained for us in Scripture, the doctrine was then formulated in a clearer creedal form. Not of course that it was unclear previously, scattered as the evidence was throughout the pages of the New Testament. It is hard to see how this criticism can be made absolute. Is there really a need, as one writer puts it to conceive of a "great leap" forward from personifications to persons as we move from the Old to the New Testament?
As a final caveat none of this is to deny the loftiness and mysteriousness of these matters in either Testament. I have not sought to make the case by marshalling all the relevant passages but to argue that it is possible for presuppositions to filter out doctrinal options unecessarily. All of this is simply to suggest that just maybe when David wrote of the LORD speaking to his Lord that he knew in a clearer way than we might have given him credit for just who he was talking about.