The multitude must needs come together. The verb is a verb neuter, as if they should say, the multitude must of necessity come together. For it had been an absurd thing that an apostle, of such rare report, should not come before the whole multitude of the faithful. For if he had eschewed the light and sight of people, the sinister suspicion might have been increased. Nevertheless, we see how modestly the elders behaved themselves in nourishing concord, when as they prevent the offense of the people in time, saving that they bear too much [1] peradventure with their infirmity, in requiring a vow of Paul. But this moderation must be kept in the Church, that the pastors be in great authority, and yet that they rule not proudly as lords, neither despise the rest of the body. For the distinction of orders, which is the bond of peace, ought not to be any cause of dissension.

Footnotes

1 - "Nimis indulgent," are too indulgent to.

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.

The multitude must needs come together - Whether this refers to a regular convocation of the Church, or to a tumult that would infallibly take place when it was heard that the apostle was come, we cannot pretend to say; but it is evident that James and the elders wished some prudent steps to be taken, in order to prevent an evil that they had too much reason to fear.

What is it therefore?.... Is it true or not? or what must be done in this case? what method must be taken to remove these objections, and reconcile the minds of the people? the multitude must needs come together; either the whole church at Jerusalem, or the great confluence of people, even of believing Jews, that were come from all parts thither; there is no hindering of their coming together, to see the apostle, and to hear what he has to say to the objections against him, and complaints of him;
for they will hear that thou art come: this can never be kept a secret, and as soon as they hear it, they will flock in great numbers; they will come open mouthed, and be loud in their complaints, and it will be difficult to pacify them; there is danger in the case, the consequence may be bad; and therefore something must be done, to remove the opinion they had formed of the apostle, and the prejudice they had entertained against him; and therefore what follows is advised to.

What is it therefore - What is to be done? The multitude must needs come together - They will certainly gather together in a tumultuous manner, unless they be some way pacified.