Religion operates on the principle of "I obey -- therefore I am accepted by God." The basic operating principle of the gospel is "I am accepted by God through the work of Jesus Christ -- therefore I obey." As we have seen, believing the gospel is how a person first makes a connection to God. It gives us a new relationship with God and a new identity. We must not think, however, that once believing it, the Christian is now finished with the gospel message. A fundamental insight of Martin Luther's was that "religion" is the default mode of the human heart. Your computer operates automatically in a default mode unless you deliberately tell it to do something else. So Luther says that even after you are converted by the gospel your heart will go back to operating on other principles unless you deliberately, repeatedly set it to gospel-mode.
We habitually and instinctively look to other things besides God and his grace as our justification, hope, significance, and security. We believe the gospel at one level, but at deeper levels we do not. Human approval, professional success, power, and influence, family and clan identity -- all of these things serve as our heart's "functional trust" rather than what Christ has done, and as a result we continue to be driven to a great degree by fear, anger, and lack of self-control. You cannot change such things through mere will-power, through learning Biblical principles and trying to carry them out. We can only change permanently as we take the gospel more deeply into our understanding and into our hearts. We must feed on the gospel, as it were, digesting it and making it part of ourselves. That is how we grow.
From The Prodigal God by Timothy Keller, pp. 114-115