Viewed in religious-historical terms, the Christian faith is a relatively late development within the the general human question of the Divine. As investigated particularly by the phenomenology of religion at the outset of the 20th century (Rudolf Otto, 1869–1937), in a universal sense the notion of God has its origins in the experience of the Holy. The Holy is expressed as that which is experienced as alarming and inspiring at the same time. In this ambiguity, for humankind it represents an overwhelmingly powerful mystery (‘mysterium tremendum et fascinans’). In this way, the religious view of the Divine comes about as a reaction to the extreme experiences – positive and negative – of (human) life, and as a response to the existential challenges that these experiences entail. The specifically Christian image of God has its roots in the religious experience of the Jewish people. The most consummate articulation of the Jewish experience of God can be found in the statement of the Old Testament Book of Exodus (3:14), in which God reveals Himself in direct speech: ‘I am who I am’. Properly understood and translated, this sentence reveals the core of the Judeo-Christian concept of God. In the Hebrew original, however, the statement, ‘I am who I am’ does not describe a static-atemporal self-identity in a metaphysical sense. The verbal form used in the statement has a powerfully future-oriented significance, in the sense of ‘I am the one who will be there [for you]’. This is a statement in which God speaks to humankind of Himself, disclosing His inner essence in these words. This assumes that God Himself must have personal characteristics, as only a person has the ability to communicate through language. This dual repetition of one and the same statement of essence expresses the absolute self-identity, independence, and sovereignty of God in contrast to everything else. It follows from this that God’s personal self-revelation to humankind is a determination of His absolute freedom, and thus a self-revelation that does not necessarily have to take place. That God’s being is the object of His free disclosure of self also makes it clear that this sovereignty also and particularly relates to an understanding of the world as creation: the world is not necessary. Instead, viewed as the outcome of God’s creative determination, the world is contingent and finite, which is also why it might not be at all. The same applies to the dialogical relationship between the God of revelation and humankind. The personally understood God can reveal Himself to humankind through language, and can receive a corresponding reply from humankind, but He is not required to do so. If, in creation and revelation, the eternal God expresses Himself in the categories of time and history, this occurs out of freedom, mercy, and grace. Because of this emphasis upon God’s freedom, the Judeo-Christian understanding of God is heavily determined by the factor of God’s will. This means that while God is absolutely transcendent with respect to the world, in the history of this people He can be experienced as a force that opens up the future to which His revelation is directed. The foundation for this salvation history is the Covenant that God concludes with His people, as manifested in God’s Ten Commandments. God’s works down through the history of salvation do not annul His absolutely steadfast self-identity. On the contrary, God’s permanent transcendence is prerequisite to His future-directed action of salvation: it is only because God remains unwaveringly true to His promises for all eternity that the temporally situated person of faith can rely unconditionally upon these promises. Already in Judaism (Old Testament), fulfilment of salvation is linked to the anticipation of a Messiah. In contrast to the Jews, Christians believe that this Messiah has appeared in an historically specific shape, in the person of Jesus of Nazareth (New Testament). In the annunciation of Jesus, the Jewish concept of the God of the Covenant is extended through an experience of God as a loving Father; in principle, this opens up the concept for dissemination beyond the Jewish people. In Jesus’ path through life, it is shown that the love that God the Father has for humankind accompanies us even to the depths of God-forsakenness and death (on the cross), even in situations that seem hopeless granting human beings a new beginning originating in the power of Divine life (mercy, forgiveness, resurrection). Through the annunciation and life of Jesus, love reveals itself unsurpassably as the most profound characteristic of God’s essence. Because of this, Christians believe that Jesus is the son of God, God become man (incarnation), in His Divine nature coessential with the Father. In the encounter between early Christianity and ancient Greek philosophy, the Christian concept of God – which in the biblical scriptures had articulated itself primarily in the form of parables and reports of experiences – was moved to speculative ideas. The content of the resulting Christian theology are of Judeo-biblical origins, while the concepts applied derive primarily from the tradition of Greek philosophy. The classical Christian doctrine of the Divine Trinity emerged from this synthesis of Judeo-Christian belief in God, on the one hand, and philosophical-Greek terminology on the other. The doctrine of the Trinity holds that before all time and creation, Christ pre-existed as divine Logos in unity with the Holy Spirit in God the Father. This expresses that the reality of the Christian God is a dynamic process of absolute love-life (inner Trinitarian generation) and absolute communication (Divine Word). Over the course of the history of thought, numerous metaphysical terms from philosophical theology were incorporated into the Christian understanding of God (e.g. absolute, necessary being; absolute intellect; absolute reality; first cause; infinity; coincidence of opposites). Although this created the conditions for the spread and better argument for the Christian belief in God, it also entailed a danger that the original, Christian experience of God be alienated by categories extraneous to it (the problem of the Hellenization of Christianity). The dimension of the Christian notion of God relating to personal revelation and the history of salvation, for instance, is difficult to conceptualise using the categories of Greek philosophy, as the latter either lacks the corresponding concepts or even judges these in a manner contrary to the genuinely Christian view. The abiding task for Christian theology, then, is to refine the notions incorporated from philosophy in such a way as to permit expression of the specificity of the Christian experience of God. The elements of the understanding of God as outlined thus far are common to all Christian churches and denominations. Differences exist, among other things, only with regard to the question of the possible ways of gaining knowledge of God. The Catholic doctrinal tradition adheres to the possibility of natural knowledge of the existence (and individual qualities) of God, i.e. knowledge obtained purely through philosophical reason and without reference to arguments proceeding from revelation. Some currents of Protestant theology, on the other hand, emphasise the inaccessibility of God to the epistemological powers of reason; there, God is considered knowable only if He discloses Himself through revelation (dialectical theology).