Therefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serves not for them that believe not, but for them that believe.
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Ambrosiaster
AD 400
The utterances of God are hidden beneath the veil of an unknown tongue, so that they should not be seen by unbelievers. Paul was saying that tongues are useful for hiding ideas from unbelievers. Commentary on Paul’s Epistles.
Wherefore tongues are for a sign . . . to them that believe not. Viz, to the unbelieving Jews, both here and in Isaiah xxviii, rather than to the Gentiles. This sign must therefore not be used by the faithful for vain glory.
Prophesying serveth not for them that believe not, but for them which believe. The teaching of the word of God and exhortation are a sign of the blessing with which God trains up His servants, and stirs them up to every good work (see ver3). Sign here is not the same as "miracle," for the Chaldeans worked no miracle when in their own tongue they chided the Jews; but sign stands for a symbol, and mark of reproof, teaching, and exhortation. But understand what has been said of the believing and unbelieving, as applying to them primarily and principally; for in a secondary sense tongues serve for a sign to the faithful, and prophecy to the unbelievers. Cf. vers23,25.
