This instruction parallels the instruction in 1 Corinthians 14:27. Yet there are a few important differences. Unlike tongues, prophesying is presented not merely as a possibility, but as a necessary element of the worship service. The words at the most
(1 Corinthians 14:27) are absent here. And whereas the use of tongues requires an interpreter (1 Corinthians 14:28), no requirement is given for the use of prophecy. There is only a command that the others
must weigh what is said.
Paul has already referred to the gift of weighing (or distinguishing) prophecy (1 Corinthians 12:10; see also the comments there). When he writes that the others
must carry out this task, he is most probably referring to the other prophets who are present in the worship service. Whether by prophesying or by listening and weighing, all the prophets can use their gifts in the worship service.
Why, if prophecy is of so much more value to the church than tongues, does Paul limit the number of prophets who are allowed to speak? One reason might be to ensure that only those who truly have something worthwhile to say get the chance to speak.
29 Let two or three prophets speak, and let the others weigh what is said.