"Disbelief in the gospel of grace, of course, keeps the unconverted from God. But a lack of deep belief in the gospel is also the main cause of spiritual deadness, fear, and pride in Christians, because our hearts continue to act on the basis of “I obey, therefore I am accepted.” [rather than the gospel which is "I am accepted, therefore I obey"]
a) Our failure to forgive others is not simply a lack of obedience, but a failure to believe we are saved by grace, too.
b) Our lying in order to cover up a mistake is not simply a lack of obedience, but a failure to find our acceptance in God rather than in human approval.
So we do not “get saved” by believing the gospel and then “grow” by trying hard to live according to Biblical principles. Believing the gospel is not only the way to meet God, but also the way to grow into him."
Tim Keller, Gospel Christianity