In 1 Cor 14:2, the act of speaking in a tongue is described as a process in which a person utters mysteries under the influence of the Holy Spirit:
2 For one who speaks in a tongue speaks not to men but to God; for no one understands him, but he utters mysteries in the Spirit. [ESV]
This matches the experience of the apostles in Acts 2:4, in the sense that they also spoke in other tongues as the Spirit gave them utterance:
4 And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance. [ESV]
In short: the Holy Spirit is the one inspiring the words that a person utters in an unknown tongue. The Holy Spirit is the source of the words.
But there is more to the phenomenon. In 1 Cor 14:13-15, Paul sheds additional light on what it means to speak in a tongue, by pointing out a distinction between speaking with the mind and speaking with the spirit:
13 Therefore, one who speaks in a tongue should pray that he may interpret. 14 For if I pray in a tongue, my spirit prays but my mind is unfruitful. 15 What am I to do? I will pray with my spirit, but I will pray with my mind also; I will sing praise with my spirit, but I will sing with my mind also. [ESV]
Here, Paul is telling us that there is a difference between speaking with one's mind and speaking with one's spirit (used as a synonym for speaking in a tongue). Notice also that the mind is unfruitful while a person speaks with the spirit (v14), and that tongue speaking can manifest during prayer and singing (v15).
And finally, we get to Paul's brief revelations about his personal experience with the gift (verses 18 and 19):
18 I thank God that I speak in tongues more than all of you. 19 Nevertheless, in church I would rather speak five words with my mind in order to instruct others, than ten thousand words in a tongue. [ESV]
Here we get two key observations:
- Paul spoke in tongues a lot (v18).
- Once again we see a distinction between speaking with the mind and speaking in a tongue (v19).
I hope to have made a good case for the existence of two different modes of speech according to Paul: (1) speaking with the mind and (2) speaking with the spirit (in a tongue). To me, this is undeniably clear. What is not necessarily clear however is the difference between these two modes.
Question: What is the difference between speaking with the mind and speaking in a tongue (with the spirit)?
My own speculative guess: during the process of speaking in a tongue, I believe that the parts of the brain that produce volitional speech become temporarily inactive. The person stops using the language parts of their brain to consciously produce speech, and instead, the words (in an unknown language) are spontaneously revealed to the person's mind by the Holy Spirit. This would explain why the mind is said to be unfruitful during tongue-speaking.