The true church can be recognized if it has the following marks: The church engages in the pure preaching of the gospel; it makes use of the pure administration of the sacraments as Christ instituted them; it practices church discipline for correcting faults.
–Belgic Confession (1561), art. XXIX [emphasis added]
If we’re to believe historic confessions of the Protestant church, there was once a time when church discipline was actually considered a defining characteristic of the church. It (nearly?) boggles the mind of the 21st century churchman to consider that of all the practices which might define a church, church discipline would trump the others save preaching the Word and participating in the sacraments (i.e. baptism & communion). Why not preaching, sacraments, and fellowship? or making disciples? or mercy ministry?
Leaving aside the question as to whether or not church discipline is rightly considered a defining mark, at a minimum we can agree that Christ Himself prescribed such a mark as a normative part of church life. So do our churches bear this mark as part of the call to follow the commands of Christ (Mat 18:15-20; see also 1Cor 5:1-13)? I suspect the answer would vary drastically depending on the church but I also suspect such a question is increasingly irrelevant–maybe “unrealistic” is the better word. After all, it’s darn near impossible to hit a mark when you’re not even facing it.