For there must be also heresies among you, that they which are approved may be made manifest among you.
Heresies - not merely 'schisms,' "divisions" (1 Corinthians 11:18), which are 'recent dissensions through differences of opinion' (Augustine), but also "heresies" - i:e., 'schisms become inveterate:' 'sects' (Greek, Acts 5:17; Acts 15:5). At present there were dissensions at the love feasts; but Paul, remembering Jesus' words (Matthew 18:7; Matthew 24:10; Matthew 24:12; Luke 17:1), foresees "there must be (come) also" matured secessions of separatists. The "must be" arises not from fatalistic necessity, but from sin necessarily bearing its natural fruits, which he foresees. These are overruled by Providence to the probation of character of both godly and ungodly professors, and to the discipline of the former for glory. There may be schism without secession (1 Corinthians 12:21; 1 Corinthians 12:25; John 7:43). "Heresies" had not yet its technical sense, referring to doctrinal errors: its means confirmed schisms. Augustine's rule is a golden one: 'In doubtful questions, liberty; in essentials, unity; in all things, charity.'
That ... approved may be made manifest - through the disapproved (reprobates) becoming manifested (Luke 2:35; 1 John 2:19).