The oldest word for church, the word that St. Paul himself used, is the Greek word “ecclesia,” from which we get the terms “ecclesial” and “ecclesiastical.” The word was in use centuries before the Christian church appeared on the scene. It referred to a socio-political gathering of citizens, who were called together to attend to the concerns of their city.
The term’s political associations probably had little to do with its use by the followers of Jesus. Those earliest followers probably borrowed the word from the popular Greek translation of the Old Testament, where it referred to Israel’s sacred assemblies, called together to worship or conduct business. It was natural for the first followers of Jesus, nearly all of whom were Jews, to borrow the familiar term for their assemblies.
Nevertheless, as news spread across the Mediterranean that a potential rival to Caesar had appeared, and that his followers were gathering in ecclesia, the ancient meaning of the word must surely have come to mind. That the Christians (Christ-ones or Christ-supporters) were meeting in socio-political gatherings across the empire caused the emperor and his prefects to see the church as a threat and attempt to abolish it.