What is it therefore? - What is to be done? What is it proper to do to avoid the effects of the evil report which has been circulated? What they deemed it proper to do is suggested in the following verses.
The multitude - The multitude of Jews.
Must needs come together - There will be inevitably a tumultuous assemblage. It will be impossible to prevent that. The reasons were, because the minds of the Jews were exceedingly agitated that one of their own countrymen had, as they understood, been advising apostasy from the religion of their fathers; because this had been extensively done in many parts of the world, and with great success; and because Paul, having, as they believed, himself apostatized from the national religion, had become very conspicuous, and his very presence in Jerusalem, as in other places, would be likely to excite a tumult. It was, therefore, the part of friendship to him and to the cause to devise some proper plan to prevent, if possible, the anticipated excitement.