I think a lot of people mistakenly think Paul said it's a turnoff for unbelievers if they walk into a meeting where they hear people speaking in tongues - they quote Paul saying, "If an unbeliever walks in, won't he think you're mad?"
But Paul didn't say it's a problem if unbelievers hear tongues being spoken at church. That wasn't the problem at Corinth - and it's not a problem today.
Think about it. The Holy Spirit gave 120 people utterance to speak in tongues in the upper room, and it was noised abroad until a whole multitude of unbelievers came together and heard them. Was that a problem? No - thousands got saved!
At Cornelius house, the Holy Spirit fell on everyone WHILE PETER WAS STILL SPEAKING, and they all got filled with the Holy Spirit - and people heard them speak with tongues. Was that indecent and out of order? God must have thought it was okay - He did it! Was it a turnoff for new people? No - it was the new people who got filled!
And at Ephesus twelve people spoke with tongues and prophesied - at once, and in public.
So why would it have been God's intention for unsaved people to see or be involved in a manifestation of God's Spirit in those three places, but a problem if unbelievers ever saw such a thing at Corinth? Answer: it wasn't a problem at Corinth - and it still isn't.
Paul's issue with the meetings at Corinth was not that unbelievers were hearing manifestations of God's Spirit such as tongues. In fact, Paul was all for manifestations of the Spirit at church! (Notice he commended them for their zeal for spiritual gifts, and encouraged them to continue coveting and earnestly desiring it; he said God had placed such expressions in the church - he even said he wished they all did it.)
Had they all spoken with tongues at once (like at Jerusalem, in Cornelius' house, and at Ephesus), Paul wouldn't have had a problem with that - even if unbelievers heard it. Because the book of Acts is full of that very thing happening.
The issue at Corinth was this: individuals seemingly were, for all intents and purposes sort of standing up, holding the floor as such, and lecturing in an unknown tongue, likely expecting the whole congregation's undivided attention while doing so; then evidently someone else would do the same, followed by someone else - with no consideration that no-one was understanding their speeches. It may have made the individuals feel good, but no-one else was getting anything out of it.
The senselessness of that was of a purely practical nature. Just like if lecturers at Uni were to lecture in a language unknown to all the students - it wouldn't educate anyone. And if the faculty persisted in doing that, you'd question their common sense wouldn't you? Of course you would - anyone would! That's purely practical.
It didn't mean Paul felt that tongues itself was a turnoff to unbelievers if they ever heard it, and that manifestations of the Spirit should therefore be avoided at church. Paul was simply pointing-out the obvious: that lecturing in a language no-one understood was impractical. But tongues itself was fine, even if unbelievers heard it, if:
If someone spoke in tongues unobtrusively; or even if everyone got filled with the Spirit and spoke in tongues at once during a meeting, without anyone holding the floor as such; or if some people did address the meeting in an unknown tongue and it was followed-up with interpretations - Paul would have been fine with that, and all of that would have been consistent with what God did in Acts; tongues would have served as an effective sign to unbelievers, as well as edifying the believers.
Paul's issue was purely practical - but it's not a problem at all if unbelievers hear tongues or see other manifestations of the Spirit at church. It can result in salvation, like in Acts. And I've seen it happen again and again!
