The understand the Mass as it was celebrated by the first generations of Christians, we must situate it in the Paschal Vigil during which the new converts received Baptism, and then participated i their first Mass. In offering the Mass they offered themselves. Every week henceforth, on Sunday, the day of the Resurrection, perhaps even every day, they are going to celebrate this same Paschal mystery in the Eucharist. The present volume contains: (1) The actual texts of the Mass as it was celebrated during the first four centuries. (2) Commentary of the Fathers on the chief Eucharistic texts from Exodus to 1 Corinthians. (3) Occasional writings, especially Sermons, in which the pastors of the 4th century expound the mystery of the Eucharist.