Acts 21:22

“What is it therefore? They will certainly hear that you are come.”

These elders knew that it was inevitable therefore that some of these prejudiced Jewish Christians would hear of Paul's arrival and probably become incensed, and angry at his presence in Jerusalem. It seemed therefore a good idea to these godly men that Paul should prove his Jewish credentials so that such people might recognise that they were wrong about Paul after all. It was a suggestion that was both sensible and helpful, taking into account the weaknesses of weaker Christians.