The Leonine and Gelasian Sacramentaries show us what is practically our present Roman Mass. How did the service change from the one to the other? It is one of the chief difficulties in the history of liturgy. During the last few years, especially, all manner of solutions and combinations have been proposed. We will first note some points that are certain, that may serve as landmarks in an investigation…Justin gives us the fullest Liturgical description of any Father of the first three centuries (Apol. I, lxv, lxvi, quoted and discussed in LITURGY). He describes how the Holy Eucharist was celebrated at Rome in the middle of the second century; his account is the necessary point of departure, one end of a chain whose intermediate links are hidden. We have hardly any knowledge at all of what developments the Roman Rite went through during the third and fourth centuries. This is the mysterious time where conjecture may, and does, run riot. By the fifth century we come back to comparatively firm ground, after a radical change. At this time we have the fragment in Pseudo-Ambrose, “De sacramentis” (about 400. Cf. P.L., XVI, 443), and the letter of Pope Innocent I (401-17) to Decentius of Eugubium (P.L., XX, 553). In these documents we see that the Roman Liturgy is said in Latin and has already become in essence the rite we still use.