What does 1 Thessalonians 1:4 mean?

Paul refers to the believers at Thessalonica as "brothers." He and his readers had believed in Jesus as Savior and thereby had become members of God's family. God was their Father, and they were brothers. God loved the Thessalonian Christians and had selected them to belong to Him.

In the New Testament, the term "brothers" is a common designation for believers. When Jesus was teaching a crowd, a man told Him His mother and brothers wanted to speak to Him. He asked: "Who is my mother, and who are my brothers?" (Matthew 12:48). Then He gestured toward His disciples and said, "Here are my mother and my brothers! For whoever does the will of my Father in heaven is my brother and sister and mother" (Matthew 12:49–50).

When the Twelve addressed their fellow believers to elect seven administrators of the daily distribution, they called the believers "brothers" (Acts 6:3). The believers in Judea who were recipients of a relief offering are called "brothers" (Acts 11:29). Paul addressed the believers at Rome as "brothers" (Romans 1:13; 7:1; 8:12; 10:1; 12:1; 15:30; 16:17). He called the Corinthian believers, Galatian believers, Ephesian believers, and Colossian believers "brothers" (1 Corinthians 1:26; Galatians 4:12; Ephesians 6:23; Colossians 1:2).