Tillich, in the complete three-volume series on Systematic Theology, addresses the overall problem of meaning and meaninglessness in modern times. Written in the middle of the twentieth century, Tillich's theology is greatly influenced by the intellectual developments of the late nineteenth/early twentieth century philosophies, including such schools of thought as phenomenology (Husserl, Heidegger, etc.) as well as existentialism, and in particular issues such as `the death of God' philosophical/theological speculations. Tillich's theology is also significantly influenced by (as are the intellectual developments of which he was part) larger historical events such as the first and second world wars. Tillich, a native of Germany, saw meaninglessness first-hand in the trench warfare of the first world war, in which he served as a chaplain. He also saw problems in the rise of the Nazi party, not just for political and cultural issues, but also theological issues (such as the idolatry of the state over God).
Tillich, spirited out of Germany during the rise of the Nazi power, spent the remainder of his career teaching in universities and seminaries in the United States. This second volume of his major work in Systematic Theology was produced in 1957, while he was teaching in the United States - it is dedicated to his friends at Union Theological Seminary, where he first taught after leaving Germany.
In Tillich's first volume of this series, he discusses the sources of theology as he sees them - scripture (both text and the events behind the text), the overall church history and tradition, and the wider traditions and history of religion in the world. Tillich has a problem with seeing experience as a source, but rather prefers this to be seen more appropriately as the medium through which the sources are understood and analysed. Tillich introduces norms and the rational character of systematic theology - Tillich is in many ways writing for philosophers who have discounted the validity of theology in the modern world; by emphasising the aspects of reason and logic in his system, he carries more weight in that community. Tillich also develops his famous Method of Correlation, a dialectical system of engagement between the temporal situation and the eternal in an ongoing process.
Tillich explores the various aspects and relationship of reason and revelation, including ways of trying to make sense in a rational manner of revelations, including what constitutes final revelation. From here, Tillich proceeds with his ontological constructions - one of the keys to Tillich's overall theology is contained here, in which God is the `ground of being'. Some have accused Tillich of being an existential atheist, because they have heard that Tillich claims that God does not exist - while it is true that, for Tillich, God does not exist, it is not true that there is no God; Tillich defines the term `existence' as being `that which is created', and as God is not a created being, God cannot exist. Rather, God is something greater, something deeper - the ground of being. God also becomes the only appropriate `ultimate concern' (another key element in Tillich's theology) - that concept is developed in that volume as well.
Volume two is primarily Tillich's Christology. Tillich has a small section that relates the second volume to the first, and restatements some major points from the first volume, but very quickly jumps into the concepts of existence/existentialism and Christian theology, developing from there concepts of sin and human estrangement (setting the stage for Christ and salvation/redemption in the new being of Christ). For Tillich, the central question of the age is one of meaning, and Christ is meaningful, as a New Being, who has a uniqueness and a universality, but not in typical Christian theological ways.
Tillich's third and final volume addresses topics of life in the Spirit, how his overall theological constructs of God as Ultimate Concern and the Ground of Being, translated through humanity's estrangement and redemption through Christ as the New Being, can have an impact on our own lives. Tillich develops ideas of self-actualisation and self-creativity, spiritual presence in faith and love, spiritual presence manifested in historical situations, and the various ambiguities. This is a perfect point at which to discuss dogmatic issues such as Trinitarianism, a problematic construct even for the most rational and traditional of theologies. Tillich concludes with a discussion of eschatology, the idea of `end-times' (not to be confused with the sort from `Left Behind' novels) and how the kingdom of God is present in reality past and present. This also approaches topics such as immortality and eternity - Tillich states that Christianity has traditional seen individual participation in eternal life in terms of immortality and resurrection, but that in fact immortality is not a biblical term - it is a Platonic idea taken on board by the church.
Tillich's theology is continued in two other volumes, the first volume produced in 1950, and the third volume in 1963, a few years before Tillich's death in 1965. Taken together, the three volumes represent a major theological force in the twentieth century, and one that is bound to continue to have impact for generations to come.