Why were the Jewish believers in Jerusalem called an ekklesia?
They were called an ekklesia because the word means a called-out assembly. The Jewish believers in Jerusalem had separated themselves from the ordinary Judaizers, making them a distinct called-out group. The context identifies them as an ekklesia, though not yet the Body of Christ. The term stresses separation, not church offices.
Les explains that Ekklesia, in its true translated form, means a called-out assembly. It does not necessarily mean a group with pastors, bishops, and deacons, although when Paul speaks of the Body of Christ in the local church, it can. He points out that Stephen used the same term in Acts 7 for the Church in the wilderness, which was not a Church but a called-out assembly because God called Israel out of Egypt. In the same way, the group of Jewish believers in Jerusalem were a called-out assembly: they had separated themselves from the run of the mill of Judaizers, so they were an Ekklesia. Yet Les maintains they were not yet the Body of Christ. He also notes that in Acts, the rioting mob in Ephesus was called an Ekklesia, and the King James translators used the word Assembly for it, showing the term can refer to any called-out assembly.