The first manifestation of speaking in tongues in the NT shows that it is a supernatural gift from the Holy Spirit for someone to speak in a real language they do not know or had never learned... And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language.(Acts 2:6)
It helps to understand how I approach a reading of these mentions of it in the NT. Actually, Paul's account should be taken as the earliest mention of it, not what Luke wrote in Acts many years later. I see Paul's speaking of it around 53 CE, in direct response to his actual encounter with an actual practice within a contemporary church of his time he had direct dealings with, as opposed to Luke's later story of something he had no direct experience of, written at least 50 years after the supposed event of the Pentecost event around 34 CE.
Luke's story reads very much like the telling of myth, at the very best it is not a firsthand account, and it stands as starkly different than Paul's where he is directly speaking to not only what he was seeing happening in his time in Corinth, but he was additionally speaking from his own personal experience as someone who spoke in tongues himself. This is a case of radically different witnesses and authority of the real events happening here. Paul's account is in fact earliest and first hand, not Luke's later telling of a story from who-knows what source, with who knows what embellishments. I definitely read Luke's story as mythic, and it does not fit any credible example the common phenomenon of glossolalia in religious practices. Miracle language acquisition is mythic, like Neo in The Matrix learning how to fly a helicopter or learning Kung Fu after it being uploaded to his brain from a computer in 5 seconds flat.
So according to the scripture example the gift of speaking in tongues is speaking is the ability given by the Spirit to speak in a real, but unlearned language, This rules out gibberish, stammering, stuttering, or repetitious sounds and much of what passes for tongues today.
But what I mentioned in the earliest mention of it in 1 Corinthians by Paul clearly puts it outside the "intelligible language" camp. He explicitly calls it speaking "mysteries" by the Spirit. In other words, they are not known languages where anyone can understand them. He explicitly states that they "build up" the person doing it. It's not because that person understands the linguistic meaning of the words. It's because they are speaking "to God", "mysteries" "by the Spirit". The activity itself engages the person on a spiritual level between God and themselves, and as a result they are "building themselves up", being edified by the spiritual activity itself, like prayer does. (Prayer is not for God, nor for the benefit of others, but for the benefit of the person who is doing the praying).
Paul explicitly says, "For if I pray in a tongue, my spirit prays, but
my mind is unfruitful." The spirit is edified, but the mind is not. It is the activity itself that builds one up. His entire point is that in the congregation, it needs to be focused on building each other up, and you speaking in tongues in ecstatic religious experience fails to do anything for others. He says, "Otherwise when you are praising God in the Spirit, how can someone else, who is now put in the position of an inquirer, say “Amen” to your thanksgiving, since they do not know what you are saying? You are giving thanks well enough, but no one else is edified."
The purpose is also shown in Acts 2:11... "we hear them speaking in our own tongues (languages) the wonderful works of God.”, then in 1 Corinthians 14;22, Paul says..."Therefore tongues are for a sign, not to those who believe but to unbelievers". This lines up with the first manifestation of the gift of tongues in the New Testament ( Acts 2:1-13), when there were unbelieving Jews present...dwelling in Jerusalem Jews, devout men, from every nation (Acts 2:5). So the purpose of the gift of tongues is to speak of the wonderful works of God and a sign to unbelievers.
This is curious since Paul says, " 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?" That doesn't sound like it will convince them of anything other than you are mad!
He then continues, "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". In other words, if someone is speaking inspired thoughts from the heart, in known languages, it does much more to convince the heart of another than pulling magic rabbits out of hats.
As you have pointed out, anyone who speaks in a tongue speaks not to people, but to God. (1 Cor. 14:2) Paul elaborates this further in chapter 14 verses 14-17. So speaking in tongues consists of praise, prayer, thanksgiving and singing addressed to God Himself about His marvelous works, mercy, and blessing to the human race.. This is very important when judging whether the gift that we hear all around today is the true, biblical gift.
When people enter into a state where they express these things in glossolalia, it is all of the above. But it's not for the benefit of others, but the one praying.
One of the first questions to ask is, "Was it addressed to men?" Often today, when interpretations are given, the content makes clear that they are claiming to give exhortation, teaching or preaching to the audience. But that is not the gift of tongues, because, according to the Word of God, this gift is not addressed to men.
Oh yes, the whole "interpretation" thing!
Yeah. Color me utterly unconvinced. What I find interesting is how many of these "interpretations" come out in King James English.
Way back in the day, I was like "huh?" Don't get me started.
"Yeah, verily I say unto thee.... I have heard thy prayers, etc."
Lastly, there is not a single instance in the New Testament of any private use of tongues, or any of the gifts of the Spirit.
Except of course for the verses I cited.
Every occurrence is in a public place or open meeting...the manifestation of the Spirit is given to each one for the profit of all (1 Corinthians 12:7).
Except of course for where Paul says explicitly, "He that speaks in an unknown tongue edifies himself", 1 Cor, 14.4. Very clearly, you do have private practice here being spoken about.
Therefore the biblical injunction for interpretation and understanding...
In the congregation, which is the whole point of what Paul was talking about in 1 Cor. 14. "In the church". He was contrasting it with private practice.