The people who believe. In the New Testament, it specifically refers to people who believe in Jesus as Lord and follow him (Acts 5:14).
One would expect the term “believers” (sometimes translated as “faithful”) to be a title for Christians since the New Testament stresses belief in Jesus. Although New Testament authors emphasized believing, they rarely used the term “believer” as a name for Christians.
There are a few clear examples of "believer" being used as a name for Christians in Acts 4:32, 10:45, 19:18, and 1 Timothy 4:12. But in other places, the term is a description, not a name (Acts 2:44; 15:5; 18:27; 1 Timothy 4:3). As a name, “believer” points to the personal commitment of Christians to Jesus. Christians were called not merely to believe something but to give themselves to someone.