Now concerning brotherly love you have no need for anyone to write to you, for you yourselves have been taught by God to love one another,
9 As to brotherly love. Having previously, in lofty terms, commended their love, he now speaks by way of anticipation, saying, ye need not that I write to you. He assigns a reason — because they had been divinely taught — by which he means that love was engraven upon their hearts, so that there was no need of letters written on paper.
In these words the apostle mentions the great duties, I. Of brotherly love. This he exhorts them to increase in yet more and more. The exhortation is introduced, not with a compliment, but with a commendation, because they were remarkable in the exercise of it, which made it less needful that he should write to them about it, Th1 4:9.
Commenting on 1 Thessalonians 4:9-12
But as touching brotherly love,.... Another branch of sanctification; which is distinct from love to God and Christ, though it always accompanies it, and from love to all mankind; and is what is peculiar to brethren in a spiritual relation, and ought to be universal, fervent, and sincere, and as Christ has loved them: concerning which the following things are said, ye need not that I write unto you.