[Righteousness] is given “freely” ([Romans] 3:24). This is very important, because it is possible to think of faith as a kind of “work,” a calling up of some psychological feeling about God. Some people think of faith as an intense attitude of surrender or a state of certainty or confidence. But Paul takes care to say it comes “freely”…We must not fall prey to the subtle mistake of thinking that our faith actually saves us, as though in the Old Testament God wanted obedience to the law for salvation, and now he has changed the requirements and all he wants is faith. That is a misunderstanding of both the Testaments, of the role of both law and faith! In both the Old and New Testaments, it is the work of Christ that merits our salvation. In both, faith is how it is received, and that is all it is. Faith is simply the attitude of coming to God with empty hands. When a child asks his mother for something he needs, trusting that she will give it, his asking does not merit anything. It is merely the way he receives his mother’s generosity.
This is crucial because, if you come to think that your belief is the cause of your salvation, you will stop looking at Christ and start looking at your faith. When you see doubts, it will rattle you. When you don’t feel it quite as clearly or excitedly, it will worry you. What has happened? You’ve turned your faith into a “work”! Faith is only the instrument by which you receive your salvation, not the cause of your salvation. If you don’t see this, you will think you have something to boast about: The reason I am saved is because I put my faith in Jesus. This is a subtle misunderstanding which cuts away our assurance, and boosts our pride. And [Romans 3] verse 27 says the gospel leaves no basis for boasting.
(Timothy Keller, Romans 1-7 For You, p 81).