"in the Spirit"

[Esaias]

The context of the verse in question is the meeting, and Paul says that we are to prefer prophesying to tongues. He does not limit "in the spirit" to only unknown tongues as the surrounding verses show.

Nowhere is praying in the spirit defined as "praying in tongues". Although praying in tongues is praying in the spirit, the reverse is not true.

For example: Although all genuine miracles are works of the Spirit, not all works of the Spirit are miracles. Although all prophesying is speaking in the Spirit, not all speaking in the Spirit is prophesying.

So the apostle rightly states that if he prays, sings, or speaks in tongues, then he is praying, singing, and speaking in the spirit, because all those activities are done "in the spirit". But it would be an error to say all speaking, praying, or singing in the spirit must necessarily be in tongues.

Further, "my spirit prays but my understanding is unfruitful" does not mean ALL praying in the spirit is unintelligible, otherwise it would mean all praying that is understandable is done WITHOUT THE SPIRIT, which is absurd. For if that were true, no praying would be useful or acceptable unless it was wholly in unknown tongues! What he is saying is plain: if he prays in tongues he is praying in the spirit, BUT his understanding is unfruitful.

Notice the ONLY passage in all the bible that talks about praying in tongues is in the heart of a teaching on the superiority of prophesying to tongues, surrounded by instructions for tongue-talker to PRAY FOR INTERPRETATION.