“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,” -Romans 4:4–5
It’s a good time to remember that Paul is not talking here about how to live our lives as Christians. That will come later, in chapters 12-16. He is talking here about how to be saved, how the ungodly can be justified and counted righteous.
He argues from the understanding that the one who works is called a worker. Workers are given wages, earnings for what they have done. And as Paul has already demonstrated, we cannot earn merits with God by our works of the law because, as he will eventually demonstrate, “…the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord” (Romans 6:23).
Additionally, if it were to be called a wage, it couldn’t be called a gift because a wage signifies something is owed; and that makes men mercenaries and God beholden to them. Instead God counts our belief as righteousness imputed to us. Our belief, or faith, is not meritorious. Faith is simply the instrument given to us by God that is used so the righteousness can be imputed. Or, as Charles Spurgeon explained, faith is the hands of the underserving beggar used to grab onto the alms offered by the one with something to give.
Martin Bucer further explains the whole of the passage this way: “If one merits any thing by his work, what is merited is not freely imputed to him, but rendered to him as his due. Faith is counted for righteousness, not that it procures any merit for us, but because it lays hold on the goodness of God: hence righteousness is not due to us, but freely bestowed.”