Uninterpreted tongues are a sign of judgment for unbelievers, interpreted tongues or prophecy can serve as a sign that might lead to repentance (1 Cor 14:21–25). Uninterpreted tongues are a sign to the unbeliever in the same way that Assyrians were a sign to unbelieving Israelites as they tramped through Israel’s streets speaking their foreign language—condemning evidence of God’s judgment for disobedience (1 Cor 14:21; quoting Isa 28:11–12). Paul follows this with an argument that appears to contradict itself. He calls tongues a sign for unbelievers not for believers (1 Cor 14:22) but then goes on to describe prophecy functioning as a sign for unbelievers (1 Cor 14:23–25).
So you see that speaking in tongues is a sign, not for believers, but for unbelievers. Prophecy, however, is for the benefit of believers, not unbelievers. Even so, if unbelievers or people who don’t understand these things come into your church meeting and hear everyone speaking in an unknown language, they will think you are crazy. But if all of you are prophesying, and unbelievers or people who don’t understand these things come into your meeting, they will be convicted of sin and judged by what you way. As they listen, their secret thoughts will be exposed, and they will fall to their knees and worship God, declaring, “God is truly here among you.” (1 Cor 14:22–25).
Here Paul uses the word ‘sign’ to mean ‘sign of God’s attitude’ (whether positive or negative). Tongues that are not understood by outsiders are certainly a negative sign, a sign of judgment. Therefore Paul cautions the Corinthians not to give such a sign to outsiders who come in.178
When the church gathers and unbelievers will be present, an understandable message is mandatory. If unbelievers don’t understand, they’ll only accuse you of being out of your minds, blaspheming the Spirit by ascribing its actions to drunken men (1 Cor 14:23). But if they heard prophecies, and perhaps a word of knowledge or word of wisdom, conviction and repentance might arise in their hearts as the Spirit laid it bare (1 Cor 14:24–25). And of course, even if unbelievers are not present, the general requirement for interpretation still rules, so that others will be edified.