When we reach Genesis 12, we step into a new stage of God's salvation-history in Scripture. Here we have the beginning of God's plan to choose and fashion a people of his own, through which to bless the whole world. Gen. 12:1-3 gives us God's call to Abram (later to be called Abraham): "I will make you into a great nation, and I will bless you... And all peoples on earth will be blessed through you." These verses, and particularly the final sentence, struck the early Christian church as being so clearly about Jesus, that the apostle Paul himself called Genesis 12:3 "the Gospel"! He wrote, "Scripture foresaw that God would justify the Gentiles by faith, and announced the Gospel in advance to Abraham: 'All nations will be blessed through you.'" (Gal. 3:8)
And, as we consider the history of God's plan, as revealed in the Bible, it becomes very clear that this verse, proclaimed to Abraham somewhere around 2000 BC, was indeed about Christ. What other event in all of Scripture could possibly be construed as the fulfillment of this promise, of Abraham's offspring blessing all the nations of the world? Not the exodus, not the giving of the OT Law, not the Davidic kingship, not the ministry of the prophets--nothing, in short, is so clear a fulfillment of this promise as the opening of God's covenant-family to the Gentile nations of the world, something that only happened through Jesus Christ. And right here, two thousand years before Jesus came, it was already being proclaimed to Abraham.
In a parallel passage in Gen. 13:14-17, God makes a further promise to Abram and his "seed" (some modern translations render this as "offspring"). This seed is both singular (which Paul notes as a direct indication that it is pointing to Christ, a use of "seed" that parallels with the Messianic prophecy in Gen. 3), and, at the same time, plural--"like the dust of the earth." Here we have the promise of Christ (the Seed) and of the church (the dust of the earth--the children of God who enter the covenant-family because of Jesus).
Two other minor points in chapter 12 are also worth reflecting on. The first point, which arises in v.7, will become a regular theme of our reflections on "Jesus in the Old Testament"--it says here that "the Lord appeared to Abram." As we'll see at multiple points in Abraham's story, these divine appearances are not metaphorical; they refer to an event in which God actually, visibly appears to Abraham and then interacts with him. Likewise, similar divine appearances of "the Lord" or "the angel of the Lord" (the latter of which is, as we'll see, almost always identified as "the Lord" himself) appear all throughout the first seven books of the Bible. The church has consistently interpreted these divine appearances as pre-incarnation manifestations of Jesus himself--that is, of God the Son, the Second Person of the Trinity.
Why would these appearances need to be read as manifestations of Christ, rather than simply as God the Father (or as God considered in the abstract, apart from any Trinitarian identification)? Because they present an otherwise insoluble problem in Old Testament theology. You see, the Bible is remarkably clear, in multiple places, that God cannot be seen, both as a matter of fact (as spirit, he is invisible) and as a warning: that if God were to be seen, the human observer would be destroyed by the unbearable weight of an encounter with the divine (see Exodus 19:21, 33:20; Judges 13:22; Job 9:11; John 1:18; 1 Tim. 6:16). And yet, despite this clear teaching of Scripture, God visibly appears, over and over again--sometimes, within the same chapter that declares that God cannot be seen.
So what do we make of this conundrum? The solution is given in John 1:18--although God the Father cannot be seen, the Son has made Him known to us. Christ is, in the words of the Apostle Paul, "the image of the invisible God." John describes him as the Logos, "the Word," the eternal self-expression of the Father. So when we see God appearing in a way that human beings can interact with in the Old Testament, the only biblical answer to that mystery is that this is an appearance of the Logos, of God's visible self-expression as mankind, whom we later know in the person of Jesus Christ. So, yes, this means that Christians have, for a very long time, believed that the Son of God was chatting with Abraham two thousand years before his incarnation as Jesus of Nazareth.
The final point of interest to note here is that in Abraham's story--the initial, prototypical story of God's chosen people--we already have a foreshadowing of the journeys of both Israel and Christ. Just as Israel went down to Egypt and then came back, and just as the Christ-child went down to Egypt and then came back, so also we see Abraham here, immediately after God's act of choosing him, make that familiar journey to Egypt and back again. Though no doubt he did not know it, he was already tracing out the pattern of the story of God's great redemption.