14:15-25 There can be no assent to prayers that are not understood. A truly Christian minister will seek much more to do spiritual good to men's souls, than to get the greatest applause to himself. This is proving himself the servant of Christ. Children are apt to be struck with novelty; but do not act like them. Christians should be like children, void of guile and malice; yet they should not be unskilful as to the word of righteousness, but only as to the arts of mischief. It is a proof that a people are forsaken of God, when he gives them up to the rule of those who teach them to worship in another language. They can never be benefitted by such teaching. Yet thus the preachers did who delivered their instructions in an unknown tongue. Would it not make Christianity ridiculous to a heathen, to hear the ministers pray or preach in a language which neither he nor the assembly understood? But if those who minister, plainly interpret Scripture, or preach the great truths and rules of the gospel, a heathen or unlearned person might become a convert to Christianity. His conscience might be touched, the secrets of his heart might be revealed to him, and so he might be brought to confess his guilt, and to own that God was present in the assembly. Scripture truth, plainly and duly taught, has a wonderful power to awaken the conscience and touch the heart.
22. 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" (1Co 14:21). "Sign" is often used for a condemnatory sign (Eze 4:3, 4; Mt 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 (Isa 28:11, 12), unbelievers, but on them that are either in receptivity or in fact believers; it makes believers of those not wilfully unbelievers (1Co 14:24, 25; Ro 10:17), and spiritually nourishes those that already believe.