From the text in Isaiah, Paul goes on to argue that unintelligible speech is a sign not of God’s favour, but of God’s judgment! It was because the Israelites did not listen to Isaiah’s clear prophetic message that God eventually sent the Assyrians, a people of strange lips and with a foreign tongue,
to them (Isaiah 28:11). God spoke through the Assyrians, not to instruct but to confuse and terrify his people.
Likewise, tongues are sign of God’s judgment over unbelievers. Given his use of Isaiah’s prophecy, it is possible that Paul specifically has in mind the Jews who have rejected the gospel. Tongues may be God’s sign to them that the gospel has now gone out to the Gentiles (see Paul’s words to the Corinthian Jews in Acts 18:6). Yet 1 Corinthians 14:23–25 seems to be broadly referring to all unbelievers.
Whereas tongues are unintelligible and confusing, prophecy is clear. Through prophecy, God comes to his people in love and mercy, instructing them and imparting faith to them. Therefore, prophecy can be called a sign for believers.
Believers include those who for the first time put their faith in the gospel (see 1 Corinthians 14:24–25).
22 Thus tongues are a sign not for believers but for unbelievers, while prophecy is a sign not for unbelievers but for believers.