It is not mentioned by the Apostolic Fathers or the Apologists. The first certain example of its use in the liturgy is in that of the eighth book of the “Apostolic Constitutions”. Here it is the answer of the people to the various Synaptai (Litanies) chanted by the deacon (Brightman, “Eastern Liturgies”, pp. 4 and 5; cf. “Ap. Const.”, VIII, vi, 4). That is still its normal use in the Eastern rites. The deacon sings various clauses ofa litany, to each of which the people answer, Kyrie Eleison. Of the Greek Fathers of the fourth century, Eusebius, Athanasius, Basil, Cyril of Jerusalem, and the two Gregories [of Nazianzus and Nyssa] do not mention it. But it occurs often in St. John Chrysostom. Its introduction into the Roman Mass has been much discussed. It is certain that the liturgy at the Rome was at one time said in Greek (to the end of the second century apparently). It is tempting to look upon our Kyrie Eleison as a surviving fragment from that time. Such, however, does not seem to be the case. Rather the form was borrowed from the East and introduced into the Latin Mass later. The older Latin Fathers, Tertullian, Cyprian, etc., do not mention it. Etheria (Silvia) heard it sung at Jerusalem in the fourth century. It is evidently a strange form to her, and she translates it: “As the deacon says the names of various people (the Intercession) a number of boys stand and answer always, Kyrie Eleison, as we should say, Miserere Domine” …