Either the Catholic Church, the universal Body of Christ which transcends time and space, OR the physical buildings which mark places of worship.
The early church was familiar with church ekklesia as one of the words used to denote Israel as the "assembly" of the Lord's People. Jesus is the vital continuity between old Israel and the new, and his faithful followers were both the righteous remnant of the old and the nucleus of the new.
(The Origin of the Bible: Newly Updated by F. F. Bruce, 2020. Page 11.)
List
- The Church of the Thessalonians