MYSTERY OF THE CHURCH presents a global picture of the main themes of current ecclesiology. First, it deals with the institution of the Church and her essential nature. Subsequently the four hallmarks of the Church are described. Her unity and holiness are guaranteed by the sanctity of Christ her Head despite the sinfulness of her members. The catholicity of the Church is also examined from the perspective of Eastern Christendom. The apostolicity of the Church leads to a description of Petrine Office. The Church is seen as the instrument of salvation, and her relationship with the State and with science is investigated. Finally the Church is pictured as leading to the Kingdom of God.