15 So what shall I do? I will pray with my spirit, but I will also pray with my understanding; I will sing with my spirit, but I will also sing with my understanding. 16 Otherwise when you are praising God in the Spirit, how can someone else, who is now put in the position of an inquirer,[1] say "Amen" to your thanksgiving, since they do not know what you are saying? 17 You are giving thanks well enough, but no one else is edified. 18 I thank God that I speak in tongues more than all of you. 19 But in the church I would rather speak five intelligible words to instruct others than ten thousand words in a tongue. 20 Brothers and sisters, stop thinking like children. In regard to evil be infants, but in your thinking be adults.

21 In the Law it is written: "With other tongues and through the lips of foreigners I will speak to this people, but even then they will not listen to me, says the Lord."[2] 22 Tongues, then, are a sign, not for believers but for unbelievers; prophecy, however, is not for unbelievers but for believers. 23 So if the whole church comes together and everyone speaks in tongues, and inquirers or unbelievers come in, will they not say that you are out of your mind? 24 But if an unbeliever or an inquirer comes in while everyone is prophesying, they are convicted of sin and are brought under judgment by all, 25 as the secrets of their hearts are laid bare. So they will fall down and worship God, exclaiming, "God is really among you!"

Matthew Henry's Commentary on 1 Corinthians 14:15-25

Commentary on 1 Corinthians 14:15-25

(Read 1 Corinthians 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.