At a time when there did not yet exist a Latin liturgy, in the meaning that will have after the Council of Trente, the Preachers adopted identical liturgical books for all houses of the Order. There were several reasons for this initiative, both spiritual and practical. They hoped to unify hearts through uniformity of custom, liturgical custom included; and since they moved not infrequently from house to house, and from province to province, particularly for studies, there was an advantage in finding everywhere the same usage, the more so considering the cost of copies of liturgical books for a mendicant order.
Accordingly the Dominican liturgy is not an original production. One has to do rather with a compilation of local usages, made, of course, with great care and concern for the sobriety required by the primitive Constitutions. Father Dominique Delalande has already shown how interesting the Gradual of the Friars Preachers is. The work of compilation was completed in 1256, during the Generalship of Humbert of Romans, who played an important role in the project.
Br. Pierre Raffin, o.p. – Bishop of Metz