Not a building — a summons
Ekklesia is a calling-out. In a Greek city it was the assembly of citizens summoned to the square. The Bible never uses it for a building; it always means a people gathered. The church is not where you go — it is who, in Christ, you are.
Old people, new gathering
Because the LXX already called Israel's assembly the ekklesia (Acts 7:38, 'the ekklesia in the wilderness'), the New Testament presents the Church not as a replacement but as the same called-out people of God, now assembled around the Messiah. This continuity is part of the tradition question: the apostolic ekklesia is the body that receives, guards, and hands on the deposit.
Where This Word Decides Debates
Ekklesia frames ecclesiology and authority — who guards the apostolic deposit — touching both holy-tradition and sola-scriptura.
When This Word Study Proves Too Much
Don't read 'church building' into ekklesia, and don't over-press the ek-kaleō etymology into 'the called-out (separated) ones' as a sect proof-text — the word simply means 'assembly.'