This one's dense, and I know I'm not good at it.
For one thing, it doesn't refer exclusively to just one point in the Gospels! Jesus preached the Kingdom pretty much constantly. That's always been a bit of a sticking point for me.
Still, the Kingdom of God is among us, is heaven on earth! So we'd better start living like it, and paying attention to the heavenly (read: eschatalogical) dimension of things. After all, as St. Paul said, "The Kingdom of God is justice and peace, and joy in the Holy Spirit" (I'm sure Taizé took some minor translation liberties for the song from which I recall the verse, but the crux of the issue is the same).
Probably my most cherished reflection on the Kingdom of God comes from the classroom of Dr. John Bergsma. Unfortunately, I don't have my notes handy, and dinner just beeped (mmm, leftovers!), so you're getting this entirely from memory.
That parable about the wheat and the chaff - you know, the one in which the farmer plants wheat, and then his enemy sows weeds in among the wheat, and the farmer's servants wants to weed right away but the farmer won't let them because they might inadvertently rip up some wheat, too... long story short, when it comes to harvest time, the farmer separates the wheat from the chaff (and the sheep go to his right, the goats to his left, if you'll permit the mixed metaphor).
Dr. Bergsma drew a diagram on the board (his drawings are so simple, and so wonderfully explicatory!), of which I will attempt to reproduce a semblance here. He started with a box.
In the box, he drew little circles (the wheat).
Then little x's for the weeds/chaff:
So that's what the kingdom of God looks like right now. Many people (he, a convert and ex-minister, pointed to Protestants, but I see the same tendency in traditionalists and many, many other groups) like to pretend like the Church (the Kingdom of God upon earth) should look more like this:
But that's just what we think, what we want, not what the Jesus said. Remember the farmer's words in the parable: he didn't want to remove the chaff because some wheat might be taken away with it. He said nothing about rearranging the garden. We will always have those bad seeds in the Church, n'importe quoi (no matter what), until the end of time.
And that's part of the package. The Kingdom of God on earth is far from perfect. So are we. Remember that the next time we go to judge someone, especially someone within our beloved Church. Time's not over yet, so we must pray and put up with those x's in the midst of our beautiful little circles.