One cannot reach the transcendent Kingdom of God without participating in the struggle of the inner-historical Kingdom of God. For the transcendent is actual within the inner-historical. Every individual is thrown into the tragic destiny of historical existence. He cannot escape it, whether he dies as an infant or as a great historical leader. Nobody's destiny is uninfluenced by historical conditions. But the more one's destiny directly determined by one's active participation, the more historical sacrifice is demanded. Where such sacrifice is maturely accepted a victory of the Kingdom of God has occurred.Now, this notion of historical sacrifice is an interesting one. What is it? A personal sacrifice that not only achieves a historical aim, but also produces a personal fulfilment of the one that is sacrificed. It is then, the question of what one lives for. Tillich claims that unless you participate in the inner-historical struggle of the Kingdom of God (and let's remember that this is not the same as the Visible Church) you cannot reach the transcendent Kingdom of God, which is his way of saying that you're no good and you life is pointless. The reason is that not being involved in this struggle is based on lying to yourself. One cannot stay neutral in this struggle, one is already involved in it just by being born. However you live your life you will contribute, positively or negatively to this struggle.
Systematic Theology III, 392.
This means that "historical sacrifice" is inevitable.
"Wherever historical sacrifice and the certainty of personal fulfilment are united in this way, a victory of Kingdom of God has taken place."