Sometimes I am poor in describing so let's go to Scripture.
Galatians 2:16, "Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified."
The faith of Christ is given to the believer in Jesus Christ, the gospel spoken about Him. Once a man believes the gospel, the faith of Christ now justifies that man forever. Should a justified believer go on to good works? Absolutely, he should. But the presence or absence of good works thereafter does nothing to justify that man. A believer, once justified, is now working and laboring for the Lord and will be held accountable at the Judgment Seat of Christ. That's why going on to good works is important to a believer. Inheritance and rewards are at stake, not salvation.