The thesis seeks to clarify the development of early Christian understanding of Jesus as the pre-existent Son of God. We propose that its roots lie principally in early Christian exegesis of psalms in the light of Jesus' self-consciousness of divine sonship and divine mission. Part I reviews recent attempts to explain the development as a result of the influence of Jewish angelology and similar speculations. In Part II we argue that neither the personification of various attributes of God, including Wisdom, nor speculations about principal angels and a pre-existent messiah in Second Temple Judaism ever provided a ready-made category for viewing Jesus as a divine and pre-existent being alongside God. In Part III an examination of the Synoptic evidence for Jesus' self-consciousness of divine sonship and divine mission in the whole context of his life and teaching shows that his self-understanding was open to interpretation in terms of pre-existence. Part IV examines the early Christian use of Pss 110:1 and 2:7 against this background. We propose that the root of pre-existent Son christology is to be found in early Christian exegesis of these two messianic psalms (the catalyst) in the light of Jesus' self-consciousness of divine sonship and divine mission (the foundation). The tremendous impact left by the resurrection event and the resulting conception of Jesus "literally" enthroned to God's right hand led them to see Jesus as the pre-existent Lord and Son of God. In Part V it is argued that the pre-Pauline 'sending' formula "God sent his Son" (Gal 4:4-5; Rom 8:304; Jn 3:16-17; and 1 Jn 4:9) derives from this understanding of Jesus as the pre-existent Son of God rather than from divine wisdom christology. Once Jesus' divinity and pre-existence were confirmed through the exegesis of the psalms, Jewish wisdom traditions were helpful in drawing out the fall significance of his pre-existence in terms of his being active in creation and co-eternal with God the Father.