Barth has an awesome sermon on Luke 12:49 reflecting on Jesus’s statement that he came to kindle a fire on the earth:
Jesus used this strong word very consciously: I am come to kindle fire. Whatever gets into fire is not only changed, but it is transmuted in a manner unheard of, into something different from what it was. Wood ceases to be wood when in the fire; it becomes ashes and gas, light and warmth. Jesus meant to say: such transmutation, such radical change is what I bring and give. Just so he purposely used that other strong word: I am not come to bring peace, but a sword, the sword that brings death, that is, not just a change and an improvement in this existence with which we are acquainted, but a transition from this existence to an entirely unfamiliar one. Let us think for a moment that that which Jesus is and that which he wants, this Immanuel! God with us! is true; that it is not simply in the Bible, and spoken by a minister in the pulpit, but that it is simply true. What then? Clearly then something new begins, something as different from all that now is as ashes, gas, light and warmth are from wood, death from life.
~ Karl Barth, “Fire Upon the Earth!” in Come Holy Spirit, 118.
This notion of radical transmutation, of the supreme novum that Jesus brings about in achieving our salvation is what we are talking about whenever we talk about “apocalyptic.”