Here is another quote from Tim Keller from Center Church. It is hard to say it much better than this.
The gospel is not just the ABCs but the A to Z of the Christian life. It is inaccurate to think the gospel is what saves non-Christians, and then Christians mature by trying hard to live according to biblical principles. It is more accurate to say that we are saved by believing the gospel, and then we are transformed in every part of our minds, hearts, and lives by believing the gospel more and more deeply as life goes on (see Rom 12:1 – 2; Phil 1:6; 3:13–14).