1 Corinthians 14:16 (ESV)

16 Otherwise, if you give thanks with your spirit, how can anyone in the position of an outsider say “Amen” to your thanksgiving when he does not know what you are saying?

For the first time Paul directly addresses those church members who speak in tongues in the worship service. He uses the singular pronoun you.

In Jewish public worship it was customary to say Amen in response to a prayer or song of praise (see Psalm 106:48; 1 Chronicles 16:36; Nehemiah 8:6). This custom was taken over by the New Testament church (see also 2 Corinthians 1:20).

The word Amen means Truly or So let it be. It is used to express wholehearted agreement with someone else’s prayer. It follows that if a person does not understand the words of a prayer (literally, if he is in the place of the uninformed), he cannot say Amen to it.