Acts 21:20

‘And they, when they heard it, glorified God; and they said to him, “You see, brother, how many thousands (literally ‘tens of thousands') there are among the Jews of those who have believed; and they are all zealous for the law.”

Their response was that they glorified God. They truly rejoiced to hear of what God had been doing. And they approved of it too. Then they pointed out to Paul that there were also grounds for glorifying God in the Jewish church. Here also many thousands, even tens of thousands, had come to believe in Jesus Christ. We need not restrict this numbering to Jerusalem. The reference is to the acknowledged Jewish church as a whole in the whole region, in contrast with Gentiles. The Jewish church too was multiplying. And because they were Jewish Christians they were zealous for the Law. A Jew who became a Christian became a better Jew.

So it is emphasised that among both Jews and Gentiles the word was being powerfully effective.

But the elders then went on to draw attention to a problem, and that was that among the Jewish Christians were those who were only too willing to believe the worst about Paul.