Jesus said, “I will build my church” (ecclesia). 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.”
“Ecclesia” means a called out assembly or congregation; political (Acts 19:39); Christian (Ephesians 1:22,23); or national (Acts 7:38).
God calls the saved out of the world to congregate in worship. This is the church in the world – in it but not of it. Jesus’ church is not synonymous with Christendom (Christians as a whole).