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. For he does not mean simply what John says in his first Canonical [1] Epistle, the anointing will teach you, (1 John 2:27) but that their hearts were framed for love; so that it appears that the Holy Spirit inwardly dictates efficaciously what is to be done, so that there is no need to give injunctions in writing. He subjoins an argument from the greater to the less; for as their love diffuses itself through the whole of Macedonia, he infers that it is not to be doubted that they love one another. Hence the particle for means likewise, or nay more, for, as I have already stated, he adds it for the sake of greater intensity.

Footnotes

1 - The Epistles of John, along with those of James, Peter, and Jude, "were termed Canonical by Cassiodorus in the middle of the sixth century, and by the writer of the prologue to these Epistles, which is erroneously ascribed to Jerome.... Du Pin says that some Latin writers have called these Epistles Canonical, either confounding the name with Catholic, or to denote that they are a part of the Canon of the books of the New Testament." --Horne's Introduction, vol. 4, p. 409. On the origin and import of the epithet General, or Catholic, usually applied to these Epistles, the reader will find some valuable observations in Brown's Expository Discourses on Peter, vol. 1.

But as touching brotherly love - The "peculiar charity and affection which one Christian owes to another." Doddridge; see the notes on John 13:34.
Ye need not that I write unto you - That is, "as I have done on the other points." They were so taught of God in regard to this duty, that they did not need any special instruction.
For ye yourselves are taught of God - The word here rendered "taught of God" - θεοδίδακτοί theodidaktoi - occurs nowhere else in the New Testament. It is correctly translated, and must refer here to some direct teaching of God on their own hearts, for Paul speaks of their being so taught by him as to need no special precepts in the case. He probably refers to that influence exerted on them when, they became Christians, by which they were led to love all who bear the divine image. He calls this being "taught of God," not because it was of the nature of revelation or inspiration, but because it was in fact the teaching of God in this case, though it was secret and silent. God has many ways of teaching people. The lessons which we learn from his Providence are a part of his instructions. The same is true of the decisions of our own consciences, and of the secret and silent influence of his Spirit on our hearts, disposing us to love what is lovely, and to do what ought to be done. In this manner all true Christians are taught to love those who bear the image of their Saviour. They feel that they are brethren; and such is their strong attachment to them, from the very nature of religion, that they do not need any express command of God to teach them to love them. It is one of the first - the elementary effects of religion on the soul, to lead us to love "the brethren" - and to do this is one of the evidences of piety about which there need be no danger of deception; compare 1-John 3:14.

Touching brotherly love - They were remarkable for this; and though the apostle appears to have had this as a topic on which he intended to write to them, yet, from the account which he received of their prosperous state by Timothy, he finds that it is unnecessary to spend any time in inculcating a doctrine which they fully understood and practised. See 1-Thessalonians 3:6.

(6) But as touching brotherly love ye need not that I write unto you: for ye yourselves are taught of God to love one another.
(6) Thirdly, he requires a ready mind to every manner of lovingkindness, and exhorts them to profit more and more in that virtue.

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. The Vulgate Latin version reads, "we have no need to write unto you"; and so some copies. It seems that it was needful to write unto them about other things, as to refresh their memories with the instructions they had given them, when with them, how they should walk and please God; and to put them in mind of the commandments given them by Christ, and that their sanctification was the will of God; and particularly it was necessary to write unto them about chastity, and purity of life, whether in or out of the conjugal state; but as for brotherly love, there was no immediate absolute necessity to write about that, either about the nature of it, or to describe the objects of it, or point out instances of it, or to exhort to it in a pressing manner: the reason is,
for ye yourselves are taught of God to love one another; not merely by the light of nature, which teaches men to be kind, courteous, affable, and beneficent; nor by the law of Moses, which obliges men to love their neighbours as themselves; nor only doctrinally by the ministry of the Gospel, which frequently inculcates the exercise of this grace as a matter of great importance and consequence; nor only by the new commandment, and example of Christ; but by the Spirit of God internally in regeneration, who, according to the tenor of the new covenant, writes this law of love, and of Christ, upon the heart; and this being written upon the hearts of the Thessalonians, by the finger of the Spirit of God, whereby they were dearly directed, and powerfully taught to exercise this grace, and discharge this duty, and under the influence of the same spirit did exercise it, it was unnecessary for the apostle to write about it, and press them to it.

We should notice in others what is good, to their praise, that we may engage them to abound therein more and more. All who are savingly taught of God, are taught to love one another. The teaching of the Spirit exceeds the teachings of men; and men's teaching is vain and useless, unless God teach. Those remarkable for this or any other grace, need to increase therein, as well as to persevere to the end. It is very desirable to have a calm and quiet temper, and to be of a peaceable and quiet behaviour. Satan is busy to trouble us; and we have in our hearts what disposes us to be unquiet; therefore let us study to be quiet. Those who are busy-bodies, meddling in other men's matters, have little quiet in their own minds, and cause great disturbances among their neighbours. They seldom mind the other exhortation, to be diligent in their own calling, to work with their own hands. Christianity does not take us from the work and duty of our particular callings, but teaches us to be diligent therein. People often by slothfulness reduce themselves to great straits, and are liable to many wants; while such as are diligent in their own business, earn their own bread, and have great pleasure in so doing.

brotherly love, &c.--referring here to acts of brotherly kindness in relieving distressed brethren. Some oldest manuscripts support English Version reading, "YE have"; others, and those the weightiest, read, "WE have." We need not write, as ye yourselves are taught, and that by God: namely, in the heart by the Holy Spirit (John 6:45; Hebrews 8:11; 1-John 2:20, 1-John 2:27).
to love--Greek, "with a view to," or "to the end of your loving one another." Divine teachings have their confluence in love [BENGEL].

As touching brotherly love. This subject springs out of 1-Thessalonians 4:6.
Taught of God. The whole gospel teaches you to love one another. When you are born of God, you are his children and all brethren. As Christ loved the brethren, so must you if you follow him.
And indeed ye do. Their conduct showed their brotherly love.
Study to be quiet. The Greeks were naturally a restless people, often given to intermeddling in the business of other people.
Work with your own hands. A Christian must not be an idler. A "loafer" cannot show forth the life of Christ. We gather, elsewhere, that some brethren at Thessalonica thought the time so short until the Lord would come that work was unnecessary.
That ye may walk honestly. Becomingly in the sight of those without. It would be a reproach if the heathen could say, "This new religion makes men idle and brings them to beggary."
Lack of nothing. The necessaries supplied by labor are especially meant.

We need not write - Largely. For ye are taught of God - By his Spirit.