Thus introduces a conclusion from the scripture quotation in verse 21.

A sign is probably the better translation here. Good News Bible‘s rendering “proof” seems to conflict with verse 24, where it is Christian prophecy, in contrast with speaking with tongues, which is said to “convict” the unbeliever who comes in from outside. Phillips translates “a sign of God’s power,” suggesting that tongues are designed to impress unbelievers rather than to “convict” them.

Another possibility is that speaking with tongues is a sign by which unbelievers may be distinguished from believers, but the wording of the present verse, not for believers, but for unbelievers, does not make this clear. Paul seems to have been turned aside from his main argument by the word translated “strange tongues” (Good News Bible‘s “strange languages”) in the quotation in verse 21.

Prophecy means “the gift of proclaiming God’s message” (Good News Bible). It is inspired speaking in normal human language, as in 12.10.

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .