Book 4 of 4 - External Means by which God Invites Us into the Society of Christ and Holds Us Therein
Chapter 1 - The True Church with which as Mother of All the Godly We Must Keep Unity
7-9 - Visible Church: membership and marks
7. Scripture speaks in two ways of the church. The invisible-to-us church - all the elect -who are in God's presence by adoption; and the visible church - all on earth professing faith. "Just as we must believe... that the former church, invisible to us, is visible to the eyes of God alone, so we are commanded to revere and keep communion with the latter, which is called 'church' in respect to men."
8. Our knowledge of limited. The Lord knows those who are His - 2 Tim 2:19. But it is edifying for us to see and be with those counted as His children. So we receive others as Christians "by a certain charitable judgment" on their "confession of faith, by example of life, and by partaking of the sacraments."
9. The church is visible wherever the Word is preached and the sacraments administered. It may be we need to reckon as believers those we doubt are, if the church counts them as such. Unbelievers "in a sense belong to [the church] until they have been rejected by public judgment." But the church as a whole can confidently be named a church if the group is honoring the Word and celebrating the sacraments.