In line with a profound theological understanding of liturgy as the Church at prayer (ecclesia orans), the focus of this Element is the variegated ways in which Christians address, turn to, and worship God in their central rituals and celebrations. Surveying a representative sample of official liturgical sources from different Christian Churches, the question is asked how 'pure' the monotheism expressed in them is. For one could argue that there is some ambiguity involved, especially with respect to (i) the peculiar position of Christ, the Son of God, and God the Father in liturgical prayers, and (ii) regarding the veneration of the saints. The essential key to unlock this complex and multi-layered reality is a meticulous study of the essential doxological nature of Christian liturgy, both from a phenomenological point of view and on the basis of fine textual analyses.