5:20

Reprove: As used in the Bible, the Greek term rendered “reprove” often conveys the idea of convincing someone that he has erred. A reproof is given with the positive goal of moving a person to acknowledge and correct his mistake. One dictionary says that the word includes the meaning “‘to set right,’ namely, ‘to point away from sin to repentance.’” It is discipline that is intended to educate. At Joh 16:8, the same Greek word is rendered “give . . . convincing evidence.”

before all onlookers: Lit., “in the sight of all.” Paul apparently means that the reproof should be given before all who were aware of the sinful course. In some cases, that would include the entire congregation. In other cases, “all onlookers” could refer to a smaller group of people who were in some way affected by the wrongdoing or who knew of it. Some “onlookers” might be eyewitnesses to an event. For example, Lu 8:47 says that when Jesus healed a woman, she “declared before all [lit., “in the sight of all”] the people why she touched” Jesus. The wording and context indicate that she spoke in front of those who heard Jesus ask: “Who touched me?” There is no indication that she made this confession before everyone in the large crowd or in the city.​—Lu 8:43-47.

those who practice sin: The Greek verb meaning “to sin” is here in a form that describes continuous action. It suggests, not just a single sin, but a course of sin that is ongoing. Some other translations similarly read “those who are sinning” or “those who persist in sin.”

as a warning to the rest: Lit., “that the rest may have fear.” These words describe the purpose of a reproof given publicly, “before all onlookers.” “The rest” to whom Paul refers are the onlookers who are thus helped to have a healthy fear of sin. The reproof helps them to see the need to avoid sin and the steps that can lead to sin.