It is clear from 1 Corinthians 14 that the first-century Corinthian believers thought that the gift of tongues was the greatest of the spiritual gifts and marked out those who were truly spiritual and who could point the way to the Lord. In various ways, Paul has pointed out that speaking in tongues is not the greatest spiritual gift, at least in the corporate assembly. That is because speaking intelligible words through prophecy edifies the whole church while speaking in tongues can edify only the speaker, especially when they are uninterpreted (vv. 1–20). In today’s passage, the Apostle focuses on how tongues actually function in worship with respect to unbelievers. He does this to demonstrate to the Corinthians that tongues did not reveal the Holy Spirit in the way they thought.
Paul quotes Isaiah 28:11–12 in 1 Corinthians 14:21, citing it as a text from the “Law” because the ancient Jews sometimes referred to the entire Old Testament in that manner. The text from Isaiah occurs in the context of a warning of God’s judgment against Israel and Judah. Isaiah’s point was that the Promised Land would be overrun by gentiles who spoke foreign languages as the old covenant people of God suffered the promised curse of exile for disobeying the Lord (see Deut. 28). The Apostle then applies the Isaiah text to unbelievers in 1 Corinthians 14:22, saying that tongues are a sign for those who do not trust Christ. Apparently, Paul means that tongues are a sign of judgment when unbelievers hear them because they make the message of God unintelligible. Foreign languages signified judgment on the unbelieving Israelites who had disobeyed God, and they are a sign of judgment on unbelievers because languages the people cannot understand cut them off from the gospel. In other words, tongues do not have the meaning for unbelievers that the Corinthians thought they did. Tongues did not point out to them the power of God and the presence of true spirituality. This is confirmed in verse 23.
On the other hand, the intelligible words of prophecy can be understood by the unbeliever, and conviction of sin and resultant faith are possible (vv. 24–25). The power of God is most clearly evident where His teaching and revelation can be understood, not where they are obscured by unintelligible speech and actions. That means uninterpreted tongues are deficient.