1 Corinthians 14:14-17 Paul continued to speak sarcastically (cf. 1Co_14:16; 1Co_4:8-10) about counterfeit tongues, so he used the singular “tongue”, which refers to the fake gift. He was speaking hypothetically to illustrate the foolishness and pointlessness of speaking in ecstatic gibberish. The speaker could not understand; and what virtue is there in praying to God or praising God without understanding? No one can “Amen” such nonsense.  ~ John MacArthur – MacArthur Bible Commentary