Here, Keller addresses the question of why we fail to spot these obvious flaws in our own character and why we continue to live in a way that often robs others of joy and God of his glory. He comments:
Virtues of gifts and temperament have a corresponding “dark side” because our gifts and natural temperament are bound up with the idols that dominate any not heart filled with the gospel of grace. Without a thorough knowledge of the gospel, we look to good things—human approval and relationships, the exercise of power and accomplishment, the control of our environment and self-discipline, the enjoyment of comfort, privacy, and pleasure—and make them into pseudo-salvations. So the person who makes an idol out of human approval may be a sensitive artist, and the one who makes an idol out of power might be a courageous leader. But gifts and temperament in the service of idols—and this is our normal state—always are a mixed blessing. They have a good side—they produce virtuous behavior—but they lead the person into a corresponding sin or vice as well.
As a result, people cannot see their sins because they look only at their virtues. For example, someone may say, “I’m not abrasive, I just speak very directly.” It is true that a direct-speaking person may do good because direct, blunt comments are sometimes needed. But overall the abrasiveness is ineffective, and the person’s boldness and confidence comes to some degree from pride and a lack of love. And for this reason, many (or perhaps most) Christians do not work on the supernatural graces of the spirit that are not natural to us, and that mitigate or eliminate the dark side—the besetting sins—of our nature.