I’ve had many discussions under the title of “Christ in the Old Testament.” But perhaps the issues would be seen more clearly if we labelled the debate: “God in the Old Testament.”
And actually, the fact that those two titles sound quite different tells you everything you need to know about the dire Christlessness of modern God-talk.
We (and I include myself here in my knee-jerk western deism) imagine that there’s a bed-rock deity called “God” who is obviously the God spoken of in Genesis. And then we discuss whether the Patriarchs also knew this shadowy figure called Messiah. And we debate how 'Messianic' certain discrete verses are, and to what degree the author was aware, and to what degree the first audience was cognisant of specific promises and appearances, etc, etc. But we almost never challenge that view of “God” which we all signed off on in the beginning!
Thus from the outset God is defined as – essentially – ‘the God of monotheism’ (broadly conceived) and Christ is defined as a nuance to a more foundational divine reality. Then we spend all our time debating how clear the nuance was!
But what if... from the beginning, Elohim was not the god of Aristotle! It’s a shocking thought I know, but let’s run with it. What if... He makes all things by His Spirit and Word and says “Let us”? And what if this is not something that needs to be kept in check by a hermeneutic that expects only an omnibeing? And what if Yahweh Elohim stoops down and breathes into Adam’s nostrils and what if, under the name “Voice of the LORD”, He walks in the garden in the cool of the day and encounters the couple as a divine Person. And what if Adam and Eve weren't blind/idiots/default-unitarians?
How much clearer Adam and Eve saw God than us! Without the “benefit” of our western theistic presuppositions, they see the “very God from very God.” They don't think in that exact language, but they certainly don’t think in unitarian categories either. They think of Elohim who creates through His Word and Spirit. They think of Yahweh Elohim - the hands-on God - who breathes life into man. They think of 'the Voice of the LORD' who walks in the garden with them. And in Genesis 4:1 they think they have begotten the LORD-Man at the first attempt (the timing was wrong, but the hope was not, cf 1 Pet 1:10).
They simply didn't have a monadic sub-structure to their doctrine of God. They were not proto-Arians, labouring under a philosophical strait-jacket. To imagine that the Divine Messiah was something extra to their simple belief in “God” fails spectacularly to get at true Old Testament faith. But it does reveal some disturbing assumptions about who we think “God” is.
Who is this “God” for whom His Word/Messenger/Messiah is an addendum? Why on earth would we begin the Scriptures with that “God”? And if the primary truths about God are unitarian, is our own faith primarily unitarian, just with a Jesus nuance?
The question is deeper than “Christ in the Old Testament.” It’s deeper even than “God in the Old Testament.” It’s the question of God. Which explains why the issue can get quite heated at times. But also why it’s so crucial.