20. Them that sin—whether presbyters or laymen.
rebuke before all—publicly before the Church (Mt 18:15-17; 1Co 5:9-13; Eph 5:11). Not until this "rebuke" was disregarded was the offender to be excommunicated.
others … fear—that other members of the Church may have a wholesome fear of offending (De 13:11; Ac 5:11).