Commentaries:
Jamieson, Fausset, and Brown
Thus from Isaiah it appears, reasons Paul, that "tongues" (unknown and uninterpreted) are not a sign mainly intended for believers (though at the conversion of Cornelius and the Gentiles with him, tongues were vouchsafed to him and them to confirm their faith), but mainly to be a condemnation to those, the majority, who, like Israel in Isaiah's day, reject the sign and the accompanying message. Compare "yet . . . will they not hear Me" (I Corinthians 14:21). "Sign" is often used for a condemnatory sign (Ezekiel 4:3-4; Matthew 12:39-42). Since they will not understand, they shall not understand.
prophesying . . . not for them that believe not, but . . . believe—that is, prophesying has no effect on them that are radically and obstinately like Israel (Isaiah 28:11-12), unbelievers, but on them that are either in receptivity or in fact believers; it makes believers of those not wilfully unbelievers (I Corinthians 14:24-25; Romans 10:17), and spiritually nourishes those that already believe.
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