Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe.
Thus from Isaiah it appears that "tongues" (uninterpreted) are not a sign for believers (though at the conversion of Cornelius and the Gentiles with him, tongues were vouchsafed to confirm their faith), but mainly to condemn those who, like Israel, reject the sign and the accompanying message. So the tongues condemned those who rejected the proffered Gospel on Pentecost (Acts 2:8, contrasted with 1 Corinthians 14:13). Compare "yet ... will they not hear me" (1 Corinthians 14:21); even primitive signs fail to arouse them. '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 serveth not for them that believe not, but ... believe. It has no effect on obstinate unbelievers, like Israel, but on them that are either in receptivity or, in fact, believers: it makes believers of those not willfully unbelievers (1 Corinthians 14:24-25; Romans 10:17), and spiritually nourishes those that already believe.