If you bless with the spirit, how will he who occupies the place of the uninformed say “Amen” at your giving of thanks? 1 Cor. 14:16.
When he had said the “Amen, “and thereby finished his prayer, those who were appointed for the purpose lit the fire. Martyrdom of Polycarp (c. 135, E), 1.42.
When he has concluded the prayers and thanksgivings, all the people who are present give their assent by saying “Amen.” This word amen means “so be it” in the Hebrew language. Justin Martyr (c. 160, E), 1.185.
The president in like manner offers prayers and thanksgivings according to his ability. And the people assent, saying “Amen.” Justin Martyr (c. 160, E), 1.186.
Out of the mouth from which you said “Amen” over the holy thing, you now shout in a gladiator’s favor! Tertullian (c. 197, W), 3.90.