In the celebration of the Roman Catholic Eucharist, the first litany (the third option for the Penitential Act of the Introductory Rite) is the Kyrie eleison (Lord, have mercy). The original Greek, which is always an option, is another indicator that the church in Rome was bilingual for several centuries (Greek and Latin). This litany probably came into Rome during a period when successive popes were from the East, particularly Syria. The tropes (titles or phrases addressed to Christ Jesus) are not penitential in nature; nor does “mercy” in the liturgy denote a request to be pardoned in the way a civil official might spring a criminal from prison. “Have mercy on us” is much closer to “be who you are for us” – unconditional love, despite our sin.
The Agnus Dei (Lamb of God) is another litany brought to Rome from Antioch by Pope Sergius. Originally it would have been sung as long as it took to complete the Fraction Rite, that is, to break the consecrated bread and fill the cups with the consecrated wine and/or do the immixtio (remember that? It will be in the final!). Even today the Lamb of God invocation and call for mercy may be repeated as long as it takes to complete the Rite. Again, we call upon the Lamb of God to reveal God’s unconditional love for us (God’s mercy) in our sharing in who we — are his Body and Blood for the Life of the World.
-Rev. Andrew D. Ciferni, O. Praem.