1 Corinthians 14:22

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 ver. 3). 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. vers. 23 and 25. Vers. 23, 24. If therefore the whole church,... he is judged of all. If all speak together confusedly and noisily, they will seem to be mad; but if all teach the faith from the Scriptures and other authorities, and preach of the way to lead a right life, the outsider will be convinced of, and reproved for, his unbelief and evil life, by all the teachers and preachers.