Josh Harris has summarized some of Keller's teaching on Religious Idols from a message at the Gospel Coalition as follows:
Truth can be made an idol. Are you resting in the rightness of your doctrine rather than the work of Jesus? If so, the Bible calls you a fool. In Proverbs, "the scoffer" is a person like this. The scoffer is always sure he is right, and always disrespectful, disdainful, and mocking toward his opponents. The internet breeds scoffers, because if you're a scoffer you get more traffic to your blog.Keller points out that all forms of idolatry are really self-salvation projects, that Christians are certainly not immune to idolatry, and that the only alternative and antidote is constant return to and focus on the Gospel of Grace.
Gifts can be an idol. You can mistake spiritual gifts for spiritual fruit. Especially if you are successful in ministry, you can begin believing in justification by ministry: "I know I'm in God's will because my ministry is going well." Many of us in the Reformed world make an idol out of being a great preacher: "If I could just be a great preacher, then my life would have significance."
Morality can be a religious idol. Holiness is good, but Christians can feel like God loves them and will bless them because of their moral record.
Steve McCoy at Reformissionay has a good resource page to sample more of Keller's teaching material or you can go to his church's website.