Darryl Dash shares some thoughts from several sources on the need for preaching the gospel to believers:

When preaching to believers, preachers often proclaim truth from a text and offer a set of application points. This approach tends to assume that the issue is behavior. The result is moralistic preaching, which is never effective in bringing about lasting change.

Gospel-centered preaching sees the real issue as a failure to believe the gospel in some area. The listener may believe the gospel at an intellectual level, but there is a “sin beneath the sin” (an idol) that needs to be confronted with the gospel. A failure to live the reality of the gospel shows that we do not really believe the gospel at that point in our lives.