Charles Ellicott Commentary 1 Corinthians 14:16

"Else if thou bless with the spirit, how shall he that filleth the place of the unlearned say the Amen at thy giving of thanks, seeing he knoweth not what thou sayest?" — 1 Corinthians 14:16 (ASV)

Else when thou shalt bless with the spirit.—In this and the following verse, the Apostle speaks in the second person, as these verses refer not to his own practice, but to that of some in Corinth. Their conduct and its results are introduced parenthetically here, in contrast with what he is presenting as his own earnest desire and practice.

He that occupieth the room of the unlearned.—A better rendering is, he that is in the position of a private individual; as we might say, a “layman”—that is, the one who comes as a private person to the assembly and does not lead the prayer and thanksgiving.

How can such a person say “Amen” when he does not know what is being said? And he cannot understand if you speak in a tongue without interpreting it.

It would seem from this verse that from the earliest apostolic times, the practice has been for the congregation to join in the thanksgiving by uttering “Amen” (the Hebrew for “So be it”) at its conclusion.