11 For with stammering lips and another tongue will he speak to this people. 12 To whom he said, This is the rest wherewith ye may cause the weary to rest; and this is the refreshing: yet they would not hear.
Isaiah connects other speech with the promised rest and refreshing.
The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
Jesus describes an unseen Spirit whose effect is nevertheless heard.
1 And when the day of Pentecost was fully come, they were all with one accord in one place. 2 And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. 3 And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. 4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
Every person filled at Pentecost speaks with other tongues as the Spirit gives utterance.
Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear.
Peter calls Pentecost an observable outpouring: ‘this, which ye now see and hear.’
44 While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. 45 And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost. 46 For they heard them speak with tongues, and magnify God. Then answered Peter,
Peter’s companions know the Gentiles received the Spirit because they hear them speak with tongues.
15 And as I began to speak, the Holy Ghost fell on them, as on us at the beginning. 16 Then remembered I the word of the Lord, how that he said, John indeed baptized with water; but ye shall be baptized with the Holy Ghost. 17 Forasmuch then as God gave them the like gift as he did unto us, who believed on the Lord Jesus Christ; what was I, that I could withstand God?
Peter defends the Gentile experience by comparing it directly to Pentecost.
For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
Tongues can function in Spirit-directed prayer outside an interpreted public message.
The context is distributed gifts in the assembled body. Acts 2, 10, and 19 describe the reception sign, while 1 Corinthians 14 regulates public messages and private prayer.
Acts repeatedly uses tongues as the recognizable sign of Spirit reception. First Corinthians addresses how tongues function after reception, especially in public worship.