Theodoret of Cyrus: Eucharist is Commemoration of the One Sacrifice

So if the priesthood according to the Law also came to an end, and the highpriest according to the order of Melchizedek offered sacrifice and made further sacrifices unnecessary, why do the priests of the New Covenant perform the sacramental liturgy? It is clear to those versed in divine things, however, that it is not another sacrifice we offer; rather, we perform the commemoration of the one, saving sacrifice. The Lord himself, remember, required this of us, "Do this in memory of me," so that we should recall with insight the type of the sufferings undergone for us, kindle love for the benefactor and look forward to the enjoyment of the good things to come.

- Theodoret of Cyrus (around A.D. 393 to around A.D. 457), Commentary on Hebrews, Chapter 8, in Theodoret of Cyrus, Commentary on the Letters of St. Paul, Volume 2, pp. 169-70 (2001), Robert C. Hill translator.