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
10-16 - Church with these marks not to be forsaken: sin of schism
10. "No one is permitted to spurn its authority, flout its warnings, resist its counsels, or make light of its chastisements - much less to desert it and break its unity." 1 Tim 3:15; Eph 1:23. That would be like adultery - Eph 5:23-32.
11. It is arrogant to leave a church with true preaching and right administration of sacraments, "with which the Lord thought His church sufficiently marked." Satan seeks to either destroy right preaching and sacraments, or to "heap contempt upon them."
12. "We must not reject it so long as it retains [these marks], even if it otherwise swarms with many faults." Not all doctrines are of equal weight and importance. "A difference of opinion over ... nonessential matters should in no wise be the basis of schism among Christians." See Phil 3:15. We either have to agree on everything, or not allow any church to be genuine due to some disagreement, or allow for disagreement on "those matters which can go unknown without harm to the sum of religion and without loss of salvation."
13. Some "with a false conviction of their own perfect sanctity," don't deign to associate with anyone in whom they see any sin at all. Others, "out of ill-advised zeal for righteousness than out of that insane pride... judge that no church exists" where they don't see holiness corresponding to the Gospel preached. This IS a problem without excuse. But they "sin in that they do not know how to restrain their disfavor. For where the Lord requires kindness, they neglect it and give themselves over completely to immoderate severity." "They think no church exists where there are not perfect purity and integrity of life." But "the church is at the same time mingled of good men and bad." See Matt 13:24-30, 47-58; 3:12.
14. The Corinthians were divided by a party spirit, greedy, letting sexual sin go undisciplined, defaming Paul, acting selfishly at Communion, and questioning the resurrection. God doesn't declare them an untrue church, but calls them saints! 1 Cor 1:2. "Who, then, would dare snatch the title 'church' from these who cannot be charged with even a tenth part of such misdeeds?"
15. Sometimes the church doesn't discipline as it should, and you may find yourself taking communion with one you know to be in unrepentant sin, shouldn't you separate? 1 Cor 5:2, 11. "Even if the church be slack in its duty, still each and every individual has not the right at once to take upon himself the decision to separate.... It is one thing to flee the boon companionship of the wicked; another, in hating them, to renounce the communion of the church.... They are much more rigid than Paul. For... he does not require that one examine another, or every one the whole church, but that each individual prove himself - 1 Cor 11:28." The unworthy don't eat and drink judgment on others, but on themselves - 1 Cor 11:29. "Individuals ought not to have the authority to determine who are to be received and who are to be rejected. This cognizance belongs to the church as a whole."
16. Those who "incite defection from the church" because of her impurity do "from ill-advised zeal for righteousness... born of pride and arrogance and false opinion of holiness." The goal of discipline should be the unity of the Spirit in the bond of peace (Eph 4:3), as Augustine said. Don't forget all those who ARE righteous in the church, those grieving their sin. Realize the benefit is greater for the whole church receiving the Word and sacraments than the danger of some unworthy men undoing or besmirching that benefit.