The incarnation is a unique case of God’s acting in time. One thing to note is that if God the Son is timelessly eternal and yet incarnate in Jesus Christ, there is no time in his existence when he was not incarnate, though since he became incarnate at a particular time in our history there were times in that history before the incarnation, and times since. … The incarnation is the “projection” of the eternal God [quoting Herbert McCabe]. There is therefore no sense in talking of the eternal Song of God apart from the incarnation except to make the point that the incarnation was logically contingent. That this, there is not point to it if by this we mean there was a time when the eternal Son of God existed unincarnated … the point is, as Herbert McCabe says, there is no pre-existent Christ with a life history independent of and prior to the incarnation. There is no time when the eternal God was not Jesus of Nazareth. This, it seems, is a more widespread view amongst Theologians than I had realised; but it still leaves me with the problem: how can one reconcile that with a notion that what makes Christianity unique is its predication of the radical novelty of grace? But this also makes me wonder: have any sects lived by the belief that time itself began with the birth of Christ, and ended with the ascension? That Christ created the world he was born into (complete with built-in backstory), and that by killing Christ humanity ended that world? That we might think we are living in 2011, but 2011 is actually only a moment existing, in some refracted sense, within 4BC-AD33? [Philip K Dick believed, according to his own coiling Exegesis, that time ended in AD33 and started again in AD1974; but for complicated reasons.]
An easier belief might be: God is 'outside time' and eternal; Christ was 'inside time' and wasn't. But then you've got to dissipate the integrity of the trinity, and wars were fought and many people killed by earlier generations of Christians over that question.