There are two striking Greek words in this verse: “filadelphias” (ESV: brotherly love) and “theodidaktoi” (ESV: taught by God). The meaning of the first word points in extrabiblical Greek to the love between brothers and sisters within the parental family. Here this meaning broadens itself to the whole congregation. However different we are, we are all brothers and sisters of one another! We are all children of God the Father through Jesus Christ (see e.g., Matthew 5:16, Matthew 6:9). That is because—and here comes the second striking Greek word—we are all taught by God.
Here one may think of Isaiah 54:13a: “All your children shall be taught by the Lord.” It is this very text that Jesus quotes in John 6:45: It is written in the Prophets, And they will all be taught by God. Everyone who has heard and learned from the Father comes to me.
Through Jesus, God therefore fulfills the promise of which the prophet Isaiah was speaking. And so, the members of the congregation are led by the Spirit (see 1 Thessalonians 4:8, and especially Romans 8:15–16)—true scholars of God. Here we touch on the beautiful secret of the church. The congregation in Thessalonica practices this special Christian love (see John 15:12, John 15:17). They are an example for every church of today, wherever in the world they may be.
9 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,