The Divine Office rests on a conviction older than Christianity: that prayer belongs to fixed hours, not only to spontaneous need. The psalmist's seven times a day I praise you, the temple's morning and evening sacrifices, and the Jewish hours of prayer that the apostles are shown keeping in Acts, at the third, sixth and ninth hours, all flow into the Christian practice. By the fourth century two streams were visible and historians of the liturgy still describe the office as their confluence. The cathedral office was the public morning and evening prayer of city churches, popular, ceremonial and selective in its psalmody, with lamps, incense and fixed psalms of praise.
The monastic office, born in the Egyptian desert, aimed instead at continuous prayer, reciting the whole psalter in course, and its ideal of completeness gradually absorbed the cathedral's simpler round. The Western synthesis was fixed by the Rule of St Benedict in the sixth century, which distributed the hundred fifty psalms across a week of eight daily hours and legislated the office's shape so durably that the phrase the work of God, opus Dei, meant this and nothing else in a Benedictine house. Around the psalms stand the other constants: the Gospel canticles, Benedictus at dawn, Magnificat at evening, Nunc Dimittis at night, scripture read in course, hymns proper to each hour, and intercession, the whole forming a liturgy that consecrates no single event but the day itself.