Anyone who speaks in a language he had not known before doesn't speak to people. He speaks only to God. In fact, no one understands that person. What he says with his spirit remains a mystery.
I would like all of you to speak in other languages. But I would rather have you prophesy. Those who prophesy are more helpful than those who speak in other languages. But that is not the case if those who speak in other languages explain what they have said. Then the whole church can be built up.
Brothers and sisters, suppose I were to come to you and speak in other languages. What good would I be to you? None! I would need to come with new truth or knowledge, or a prophecy or a teaching.
It's the same with you. You must speak words that people understand. If you don't, no one will know what you are saying. You will just be speaking into the air.
Suppose you are praising God with your spirit. And suppose there are visitors among you who don't understand what's going on. How can they say "Amen" when you give thanks? They don't know what you are saying.
In the Law it is written, "Through people who speak unfamiliar languages and through the lips of strangers I will speak to these people. But even then they will not listen to me." --(Isaiah 28:11,12) That is what the Lord says.
So speaking in other languages is a sign for those who don't believe. It is not a sign for those who do believe. But prophecy is for believers. It is not for those who don't believe.
Suppose the whole church comes together and everyone speaks in other languages. And suppose visitors or unbelievers come in. Won't they say you are out of your minds?
But suppose unbelievers or visitors come in while everyone is prophesying. Then they will be shown by all who speak that they are sinners. They will be judged by all.
Brothers and sisters, what should we say then? When you come together, every one of you brings something. You bring a hymn or a teaching or a word from God. You bring a message in another language or explain what was said. All of those things must be done to make the church strong.