From Bart Ehrman’s Apocalyptic Prophet of the New Millennium:
At the same time, there are other indications, even in these earlier sources, that in some sense Jesus and his followers thought they were already enjoying aspects of the future Kingdom in the present. As a result, probably the majority of scholars have been content to say that Jesus talked about the Kingdom as both future and present.
I think this general view (and as stated, it is very general!) is right, even though I’m not confident that most scholars have understood in what sense it’s right. It should be quite clear by now that Jesus’ predictions about the coming Kingdom cannot be watered down, compromised, misquoted to death. For they form the very core of his teachings. His entire proclamation consisted in a call to prepare for the coming Kingdom, which would be brought in by a final judgment through the imminent appearance of the Son of Man. Jesus’ teaching of what we might call “ethics” was advanced to show people how they could be ready.
At the same time, since the end represented an act of God to reclaim his creation for himself, Jesus understood that God was even now, in the present, ultimately sovereign over this world, notwithstanding the fact that the forces of evil had been unleashed against it. God was still, in the final analysis, in control, and could act, even in the present, on behalf of those who followed his will. Moreover, those who followed his will in the present—who would, then, inherit the Kingdom that was coming in the future—were in some sense practicing the ethics of the future Kingdom. In that sense, they were experiencing a kind of foretaste of what life in the Kingdom would be like.
Some of Jesus’ best-known parables suggest that in some sense the Kingdom is already being experienced in the present, leading some scholars to make the mistake of thinking that there was nothing radically new to come in the future. That this is a misreading of these parables should be obvious by now. And it should be emphasized that even these parables themselves stress the enormous difference between the small and inauspicious experience of the Kingdom in the present and the enormous and cataclysmic coming of the Kingdom in the future.
Most of these parables have to do, in one way or another, with illustrating this immense difference. For example, in a parable independently attested in Mark and Thomas, Jesus likens the Kingdom to a mustard seed, that begins as a tiny seed (the “smallest on earth,” according to Mark) but then becomes a huge shrub, large enough for birds to nest in (“the greatest of all,” Mark 4:30-32). Scholars have had a field day with this parable, trying to make it mean all sorts of things…
…in both Mark and Thomas, the point is that something with a tiny beginning has such a huge result. Jesus and his followers had not exactly taken the world by storm! But when the Son of Man arrives, as they anticipate, a storm will be the least of the world’s problems. Thus the Kingdom was like a mustard seed: a small beginning in Jesus’ ministry, but an immense outcome on the day of judgment.
So, too, in the parable of the woman putting leaven in three batches of dough (Q: Matt. 13:33; Luke 13:20). The leaven is hidden at first, but it eventually permeates all three batches in their entirety. The Kingdom of God is like that: inauspicious beginnings with enormous consequences.