This is a study of how Christian Trinitarian understanding of monotheism has been expressed in Christian prayer and worship. It studies the emergence and development of a distinctive language of prayer within the New Testament and among some early Church Fathers, which laid the basis for later belief in and worship of the Trinity. This contemplative belief is studied in the light of Eastern and Western understandings of the Trinity, by examining certain key figures Christian history. It concludes with a discussion of the continuing mystical and contemplative traditions within Eastern and Western Christianity. This Element does not consider the development of Christian liturgy, however, which has already been considered in an earlier Element in this series. Nor does it address the varied impact of the Reformation upon the Western churches, which will be addressed by further Elements to be published in this series.