John Calvin Commentary Acts 21:24

"these take, and purify thyself with them, and be at charges for them, that they may shave their heads: and all shall know that there is no truth in the things whereof they have been informed concerning thee; but that thou thyself also walkest orderly, keeping the law." — Acts 21:24 (ASV)

Which they have heard of thee are nothing. They seem to persuade Paul to dissemble. For the rumor was not baseless that he called away the Jews from the ceremonies and that he was not walking in observance of the law.

But we must remember what I said just now: it was sufficient for Paul and the elders if they could remove the slander that was unjustly raised—namely, that he was an apostate from the law. A better opportunity might soon be offered, so that in clearing himself he might gradually call them back from their error.

Nor was it good or profitable for Paul to be considered an observer of the law for any long time, as the disciples then commonly were. For by this means, a thick veil would have been put before their eyes to darken the light of Christ.

Therefore, let us understand that Paul did not dissemble, but sincerely professed that he did not hate the law, but rather thought reverently of it. They wanted him to contribute to the cost with them, because they were accustomed to pool their money so that they might offer sacrifice together.