Emil Brunner writes
Christ desires no invisible army. He wants a host of such a kind that even the children of this world, who know nothing of faith nor want to know, will be able to note that there is something mightily at work within these “called-soldiers”; that they obey a mighty Other and no longer their own wills.
An invisible church is no church at all just as an invisible disciple is no disciple at all and invisible obedience is no real obedience at all. If Christianity isn’t visible, it isn’t Christianity. All those who believe faith is a ‘private’ matter wish it to be an ‘invisible’ matter and thus anything but true faith.