What does 1 Thessalonians 4:9 mean?

Earlier, Paul complimented the Thessalonian Christians' reputation for brotherly love (1 Thessalonians 1:6–10). As such, Paul sees no reason to discuss brotherly love with his readers, because they had learned from their heavenly Father that they should love another. When they had become Christians, they had received God's love. The Holy Spirit had placed this love in their hearts (Romans 5:5), and that love had spilled over to their fellow believers.

In 1 Thessalonians 1:3 Paul commends the Thessalonians for their "labor of love." Their love wasn't simply something to talk about. It was something that caused them to roll up their sleeves and work to benefit others.

Jesus set the perfect example of genuine love. John chapter 13 reports that He assumed the role of a slave in the upper room where He and His men were gathered. Verse 1 tells us "…having loved his own who were in the world, he loved them to the end." Jesus put aside His outer garments, wrapped a towel around His waist, poured water into a basin, and began to wash His disciples' feet. Later, He issued a new commandment to His disciples. He said, "A new commandment I give to you, that you love one another: just as I have loved you, you also are to love one another" (John 13:34). John, the "apostle of love," observes that love for one's fellow believer is proof that he or she has become a child of God (1 John 3:14).