Christians who know the gospel in principle and who have been changed by it nevertheless continually revert to works-righteousness and self salvation in a myriad of subtle and not so subtle ways. A basic insight of Martin Luther's was that "religion" is the default mode of the human heart. Your computer operates automatically in default mode unless you deliberately tell it to do something else. Luther says that even after you are converted by the gospel, your heart will go back to operating on the religious principle unless you deliberately, repeatedly set it to gospel-mode. This then is the basic cause of our spiritual failures, uncontrolled emotions, conflict, lack of joy, and ministry ineffectiveness. We believe the gospel at one level, but at deeper levels we continue to operate as if we are saved by our works.
Timothy Keller - Gospel in Life