Ekklesia is the Greek word translated church, but it never means a building — it means an assembly, a gathering, literally the called-out ones (from ek, out, and kaleo, to call). It was an ordinary word for a public assembly of citizens, which the New Testament fills with new meaning: the community God has called out of the world and gathered to himself.
When Jesus said I will build my church, he used ekklesia — declaring his intent to gather a people, not to erect a structure. The church is therefore first a who, not a where: those whom God has called. Wherever they gather, in a cathedral or a living room, there is the ekklesia — the assembly of Christ, against which the gates of hell shall not prevail.
Ekklesia in Scripture
And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.