1 Thessalonians 4:9

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. Thus by his good opinion of them he insinuated himself into their affections, and so made way for his exhortation to them. Note, We should take notice of that in others which is good, to their praise, that by so doing we may lay engagements upon them to abound therein more and more. Observe, 1. What it is that the apostle commends in them. It was not so much their own virtue as God's grace; yet he takes notice of the evidence they gave of the grace of God in them. (1.) It was God's grace that he took special notice of: that God had taught them this good lesson: You yourselves are taught of God to love one another, Th1 4:9. Whoever does that which is good is taught of God to do it, and God must have the glory. All who are savingly taught of God are taught this lesson, to love one another. This is the livery of Christ's family.

— Matthew Henry's Concise Commentary (public domain)

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 (Joh 6:45; Heb 8:11; Jo1 2:20, Jo1 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].

— Jamieson-Fausset-Brown Commentary (public domain)