The quality of all acts in which man affirms himself existentially has two sides, the one in which he removes his center from the divine center (unbelief) and the other in which he makes himself the center of himself and his world (hubris). The question naturally arises concerning why man is tempted to become centered in himself. The answer is that it places him in the position of drawing the whole of his world into himself.This no doubt to many nonsensical quotation is actually a very concentrated formulation of Tillich's doctrine of sin. Sin is very important in Tillich's theology, and his treatment of it is very attractive.
Systematic Theology II, 52.
To understand the quote above one need to know that Man is essentially centred in God. Thus it becomes a temptation to try to center in one-self. It is to turn away from God. The interesting thing is that Tillich connects creation and sin - they are actually the same thing from two considered from two different perspectives. For something to come into existence it has to become separated from God. But this is at the same time the tragedy of the creature, it looses its center, its union with God. This is why we experience sin as estrangement, from God, from nature and from ourselves.
In my post on sin in my Ideas for a theology of decline, I noted that in such a theology creation becomes central to the understanding of sin, but for a different reason. Since God is the creator, the ultimate form of sin must be to destroy His creation, as we are currently doing. Obviously this kind of sin is the ultimate form of estrangement - it is collective suicide and attempted deicide. Man uses his world to make himself center of the world to the point where the world seizes to exist.