Our destiny is that out of which our decisions arise; it is the indefinitely broad basis of our centered selfhood; it is the concreteness of our being which makes all our decisions our decisions. When I make a decision, it is the concrete totality of everything that constitutes my being which decides, not an epistemological subject. This refers to body structure, psychic strivings, spiritual character. It includes the communities to which I belong, the past unremembered and remembered, the environment which has shaped me, the world which has made an impact on me. It refers to all my former decisions. Destiny is not a strange power which determines what shall happen to me. It is myself as given, formed by nature, history and myself. My destiny is the basis of my freedom; my freedom participates in shaping my destiny.The problem of human freedom has caused much confusion throughout the ages and continues to do so today. Those that deny freedom today refer more to biology and "neuroscience" than strange forces, but there still seems to be a very poor understanding of what man's freedom actually means. A Finnish philosopher, Hannes Nykänen, has written that trying to prove the freedom of the will is a bit like trying to sneak a peak of oneself in a mirror. The fact is that we experience ourselves as free, and when one is denying our freedom on the basis of "science" one is simply addressing a different matter. Much confusion, yes.
Systematic theology I, 185.
The genius of Tillich's polarity of freedom and destiny is that it explains and acknowledges the limits of our freedom. This idea returns later in the system when Tillich treats Original Sin.
Based on this description of the structure of being, it seems reasonable to try to use the limited freedom one has to increase one's freedom. Obviously one can not escape one's destiny, but the choices we make "participate" in the forming of this destiny. This is valid on a cultural level too. What we are doing when we change the climate and use up the earth's resources is reducing our freedom.