Littlejohn sums up an interesting conversation:
Wilson kicked things off with an opening statement taken more or less straight out of the redoubtable Paul Avis’s magnificent Church in the Theology of the Reformers. Wilson noted that although Luther and Calvin focused their ecclesiology primarily on “defining the center” of the church, its communion with Christ through word and sacrament, from early on in Protestantism, there was a temptation to try and draw the circumference, by adding the “third mark,” discipline. Given that it is the very nature of discipline to be boundary-policing, it ought be no surprise that those who most stressed the use of discipline as a “third mark” were often quite preoccupied with defining the circumference of what counted as right discipline, and thus what counted as the church. Wilson sensibly observed that, although you won’t hold on to sound word and sacrament for very long without decent discipline (just as you mightn’t have a good garden for very long without a fence), that still leaves discipline as clearly a matter of bene esse, rather than esse.
Of course, Wilson did not note what Avis goes on to note, which is that even word and sacrament can come to be treated as circumference-markers, rather than center-definers, if you lay too much stress on the adverb “rightly” that goes along with them in the Augsburg Confession—the “word rightly preached; the sacraments rightly administered.” It goes without saying that they must be rightly preached, but if we feel like we have to decide just what counts as “rightly” to know what is and isn’t a church, then we may quickly get caught up in a lose-lose game of mutual excommunication. So I would’ve pointed out, but Wilson did only have a few minutes for his opening statements, so fair enough.