If, on the other hand, someone in the church was prophesying and the congregation was receiving instruction, both visitors would gain a positive impression from the conduct of the believers. More importantly, what the prophet said would also convict them (cf. 1 Corinthians 2:14-15). Paul"s description of the visitors" response came from Isaiah 45:14 (cf. Zechariah 8:23) and contrasts with the unresponsiveness of the Israelites to messages God sent them in foreign languages. Prophecy would result in the repentance of visiting unbelievers, but tongues-speaking would not. These verses summarize the effects of good Christian preaching on unbelievers. [source][source][source]
"The gift of prophesying, however successful, is no glory to the possessor of it. It is the Spirit of God, not the preacher"s own power, that works the wonderful effect." [1][source]
Paul did not mean that every individual in the church would either speak in tongues or prophesy (cf. 1 Corinthians 14:23). He meant that if one of those gifts dominated to the exclusion of the other the stated results would normally follow. [source][source][source]
"The Corinthians tend to shut their ears to prophecy because they gain more satisfaction from listening to tongues than from hearing their faults exposed and their duties pointed out in plain rational language." [2][source]
To summarize, Paul permitted only intelligible utterances when the church gathered for worship because they edify believers and bring the lost to conviction of their need for salvation. [source][source][source]