It is plain that there was then, as now, the practice of giving and receiving letters in commending stranger brethren to the assemblies. And a valuable means of introduction as well as a guard it is, provided we hold it in spirit, not in letter: otherwise we might fail doubly―in refusing those who ought to be received, where circumstances have hindered the requisite voucher, and in receiving those who, being deceivers, can supply themselves with any letter which may the more effectually mislead. The aim of all such provisions is to afford adequate testimony to the assembly of God, which is in no way bound to a form however excellent, if wanting, provided perchance other means of godly satisfaction leave no reasonable hesitation to those who judge fairly and in love. It is mischievous when that which God uses for our mutual comfort is perverted by legalism into an instrument of spiritual torture, as may be sometimes the lack of a commendatory note, or some kindred formality.
(From “Notes on II Corinthians; G. Morrish, London; 1882. P. 38”)