I appreciate the following comments John Calvin made when discussing Galatians 1:2b. They go hand in hand with WCF 25.4: “Particular churches… are more or less pure, according as the doctrine of the gospel is taught and embraced, ordinances administered, and public worship performed more or less purely in them.” Here’s Calvin:
We do not always find in churches such a measure of purity as might be desired. The purest have their blemishes; and some are marked, not by a few spots, but by general deformity. Though the doctrines and practices of any society may not, in all respects, meet our wishes, we must not instantly pronounce its defects to be a sufficient reason for withholding from it the appellation of a Church. Paul manifests here a gentleness of disposition utterly at variance with such a course. Yet our acknowledgment of societies to be churches of Christ must be accompanied by an explicit condemnation of everything in them that is improper or defective; for we must not imagine, that, wherever there is some kind of church, everything in it that ought to be desired in a church is perfect.