Technically, repentance is a change of mind, not a turning from sin. The Greek word translated “repentance” is metanoia, and the meaning is simply “a change of mind.” In common usage, however, we often speak of repentance as “a turning from sin.” There is a good reason for this.
Repentance is often associated with salvation in Scripture. What happens when the Holy Spirit begins His work to bring a person to salvation? The Spirit gives the sinner a personal understanding and infallible conviction that the facts concerning his spiritual state are true. Those facts are his personal sin, the eternal punishment due him for his sin, the substitutionary nature of Jesus’ suffering for his sin, and the need for faith in Jesus to save him from his sin. From that convicting work of the Holy Spirit (John 16:8), the sinner repents—he changes his mind—about sin, the Savior, and salvation.
When a repentant person changes his mind about sin, that change of mind naturally leads to a turning from sin. Sin is no longer desirable or fun, because sin brings condemnation. The repentant sinner begins to abhor his past misdeeds. And he begins to seek ways to amend his behavior (see Luke 19:8). So, ultimately, the result of the change of mind about sin is good deeds. The sinner turns away from sin toward faith in the Savior, and that faith is shown in action (see James 2:17).
The change of mind (repentance) is not precisely the same as the active turning from sin and visible performance of good deeds, but one leads to the other. In this way, repentance is related to turning from sin. When people speak of repentance as a turning from sin (rather than a change of mind), they are using a figure of speech called metonymy. In metonymy, the name of a concept is replaced with a word suggested by the original.
Metonymy is quite common in everyday language. For example, news reports that begin, “The White House issued a statement today,” are using metonymy, as the name for the building where the President lives is substituted for the name of the President himself.
In the Bible we can see other examples of metonymy. In Mark 9:17 the father states that his son has “a mute spirit” (NKJV). The evil spirit itself is not mute. The evil spirit causes the boy to be mute. The spirit is named after the effect it produces: a mute child. The metonymy here replaces the cause with the effect. Similarly, using the word repentance to mean “a turning from sin” replaces the cause with the effect. The cause is repentance, a change of mind; the effect is a turning away from sin. A word is replaced by a related concept. That’s metonymy.
In summary, repentance is a change of mind. But the full biblical understanding of repentance goes beyond that. In relationship to salvation, repentance is a change of mind from an embrace of sin to rejection of sin and from rejection of Christ to faith in Christ. Such repentance is something only God can enable (John 6:44; Acts 11:18; 2 Timothy 2:25). Therefore, true biblical repentance will always result in a change of behavior. Maybe not instantly, but inevitably and progressively.