The Reformers concluded that there were two certain marks of a true church — sound teaching and the sacraments. Church discipline is a third mark that is Scripturally-based and difficult to leave off this list.
2 Timothy 4:1-5 provides a context for understanding church discipline. In this passage are 3 calls for solemnity:
First, there is a solemn declaration to preach the Word, and be prepared to reprove, to rebuke and to exhort.
Second, there is a solemn prediction that sound doctrine will not be endured, that false teachers will be collected, that truth will be perverted, and myths will be followed.
Third, there is a need for solemn administration to be sober, endure suffering, work at evangelism, and to be committed.