In his metaphysical approach, Tillich was a staunch existentialist, focusing on the nature of being. Nothingness is a major motif of existentialist philosophy which Tillich employed as a means of reifying being itself. Tillich argued that anxiety of non-being (existential anguish) is inherent in the experience of being itself (In reference to Tillich's philosophy of art see The Scream by Edvard Munch). Put simply, people are afraid of their own death. Following a line similar to SÃƒÂ¸ren Kierkegaard and almost identical to that of Sigmund Freud, Tillich says that in our most introspective moments we face the terror of our own nothingness. That is, we "realize our mortality", that we are finite beings. The question which naturally arises in the mind of one in this introspective mood is what causes us to "be" in the first place. Tillich concludes that radically finite beings (which are, at least potentially, infinite in variation) cannot be sustained or caused by another finite or existing being. What can sustain finite beings is being itself, or the "ground of being". This Tillich identifies as God. Much of Tillich's phenomenological language with regard to being can be traced back to Martin Heidegger, with whom Tillich was in contact prior to 1933. Tillich also utilized some of the basic framework of Heidegger's fundamental ontology in the discussion on Being and God in the Systematic Theology.
Another name for the ground of being is essence. Essence is thought of as the power of being, and is forever unassailable by the conscious mind. As such it remains beyond the realm of thought, preserving the need for revelation in the Christian tradition.
Contrasted to essence but dependent upon it is existence. Existence is that which is finite. Essence is the infinite. Since existence is being and essence is the ground of being, then essence is the ground or source of existence. But because the one is infinite and the other not, then existence (the finite) is fundamentally alienated from the essence. Man is alienated from God. This Tillich takes to be sin. To exist is to be alienated.
"God does not exist. He is being itself beyond essence and existence. Therefore to argue that God exists is to deny him."
This Tillich quotation summarizes his conception of God. He does not think of God as a being which exists in time and space, because that constrains God, and makes God finite. But all beings are finite, and if God is the Creator of all beings, God cannot logically be finite since a finite being cannot be the sustainer of an infinite variety of finite things. Thus God is considered beyond being, above finitude and limitation, the power or essence of being itself.
Tillich stated that since things in existence are corrupt and therefore ambiguous, no finite thing can be (by itself) infinite. All that is possible is for the finite to be a vehicle for revealing the infinite, but the two should never be confused. This leaves religion in the situation where it should not be taken too dogmatically, because of its conceptual and therefore finite and imperfect nature. True religion is that which correctly reveals the infinite, but no religion can ever do so in any way other than through metaphor and symbol. Thus the whole of the Bible should be understood symbolically, and all spiritual and theological knowledge cannot be other than symbol. This idea is used by theologians as an effective counterpoint to religious fundamentalism.
Tillich was the greatest philosophical theologian of recent history. I am interested to hear people's response to any of the above, but particularly the section I have highlighted in bold.