1. I don’t think that Jesus had the conversion of the empire in view, though I wouldn’t rule out the possibility that he would have enjoyed a drink with Constantine, given the chance. His eschatological horizon was the war against Rome, against which background he speaks of the restoration of the people of God, not least in terms of a feast when the forgiven and healed part of Israel would sit down at table with Abraham, Isaac and Jacob (eg. Matt. 8:11). I would take the saying about drinking from the fruit of the vine (Mk. 14:25) as a reference to the theme of an eschatological banquet that symbolizes the renewed family of the patriarchs. A fruitful vine is a natural Old Testament metaphor for righteous Israel. In the context of the supper, however, there is perhaps the further thought that Jesus will specifically celebrate with his suffering disciples, who will share in his vindication and authority as the Son of Man.
2. Your second comment is entirely appropriate. For my argument at this point to be valid we would have to recognize that there is always a gap between prophetic language and the realities of history to which it refers. If we think of Jesus’ “kingdom” as an ideal state of affairs, then we will inevitably have difficulty associating it with Constantine. If we think of it more pragmatically or realistically as a reference to a foreseen transformation in the status of Israel among the nations, as I have argued, then it is not so preposterous to suggest that the empire-wide acknowledgment of Christ as Lord over the gods and man-gods of the ancient world constituted the proper historical fulfilment of the New Testament’s belief regarding what Israel’s God was doing in the ancient world.
So my argument is that our problems arise because we are trying to import a modern, a-historical evangelical idealism into Jesus’ much more earthy, pragmatic, Jewish outlook. In any case, Christendom is a problem however you look at it. If we don’t regard it as a deeply flawed expression of “new creation”, just as Old Testament Israel was a deeply flawed expression of “new creation”, we have the problem of explaining how God allowed the supposedly true church of the first centuries to degenerate so badly, to the extent that it became abusive empire.
We probably fixate too much on the imperial dimension. Europe became a thoroughly Christian society, which at many levels and in many ways endeavoured to embody the ideal corporate relationship to a creator God. We also probably think rather too highly of ourselves as evangelicals. The people of God is always deeply flawed, which is why we rely on grace.
Having said that, the cognitive gap between prophetic vision and historical reality is not accidental or merely cultural. It reflects a persistent unhappiness with the way things are in the world and a constant striving for redemption and newness. It is a generator of hope. Our life under Jesus as our king is imperfect, but my argument is that for us now the overarching hope is not for kingdom but for new creation. The kingdom came so that we might be new creation.