I have seen two lexicons and they are divided as are the commentaries. 1. The Jews were the ones that had undergone John's baptism but that doesn't mean they were the only ones. They may have been Gentile proselytes. 2. They were in Ephesus which had many Gentiles but there were Jews living there as well (Acts 20:21). 3. One commentary gave a minor argument in favor that these were Jews in that the number 12 was used in Acts 19:7. A number of importance to the Jews. I personally believe they were Jews. Any thoughts?