Now to the one who works, his wages are not counted as a gift but as his due.  And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness, (ESV)
Imagine this scene for a moment. You are sitting at your desk. It is Friday morning. Your boss comes into your tiny cubicle and brings you an envelope for your attention. You are a bit anxious, not knowing what this could be about. You open up the envelope and discover it is your paycheck for the previous week.
You are overcome with emotion. You think of all the things that you can now do with this money. Your boss has already walked away but you run after him. You embrace him and begin to thank him profusely for this amazing act of generosity. You feel so unworthy to have received such a wonderful gift of financial blessing.
Sound ridiculous? I hope so! You recognize that you get a paycheck because you worked for it. It is not an undeserved gift by a generous boss. Consider what Romans 4:4 is saying. If salvation were by works then it would be something that God owed to you. You worked for it. He gave it to you. But we know that Scripture tells a very different story.
In Romans 4:5 Paul tells us that the one who does not work, but rather believes in a God who gives gifts to the ungodly who do not deserve it, it is him who receives God's righteousness. It is amazing how we try to add in the idea of works to salvation. They do not belong there. Some will say, believing is the work of salvation. That is contrary to this verse. Believing faith is also a gift, not a work.
God does not owe you salvation. You cannot possibly do enough work to earn it. Faith is the avenue by which we receive the true gift of God's righteousness.