Why was the Ephesian mob called an Ecclesia if it was not a church?
The word Ecclesia means a called-out assembly, not exclusively a church. The Ephesian mob in the amphitheater was an assembled group, so it was called an Ecclesia even though it was ungodly and not a church. The term can describe any called-out gathering.
Les explains that Stephen called Israel in the wilderness an Ecclesia even though it was not a church as we understand church; it was Israel called out of Egypt, a called-out assembly. In Ephesus, Paul's preaching of the Gospel affected many pagans so they began throwing away idols. The silversmiths precipitated a riot, and the mob ended up in the amphitheater, which was also called an Ecclesia. It was not a church or anything godly, but ungodly, yet still an Ecclesia—a called-out assembly. Likewise, believing Jews at Jerusalem were an Ecclesia, called out of Judaism, meeting apart while still under Judaism's umbrella.