From Michael Goheen’s A Light to the Nations: The Missional Church and the Biblical Story:
[The] witness of the early church was publicly subversive. The early church did not allow itself to be pushed into a private realm in some obscure corner of Roman society. It refused to conform to the public doctrine of the roman Empire and lived out the story of the Bible instead. Its confession that “Jesus is Lord” stood in stark opposition to the confession “Caesar is Lord,” which bound the empire together. It called itself ekklesia – a public assembly called out by God as the vanguard of the new humanity – explicitly rejecting the notion of being merely a private religious community interested only in future and otherworldly salvation.
The church is not a private religious community but a public witness to the lordship of Jesus.