Rom. 4:4-5
4 Now to the one who works, his wages are not counted as a gift but as his due. 5 And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness.

Through these two verses in Romans 4, the Apostle Paul explains the nature of faith, righteousness, and grace. The key word here is "counted," used once in each verse. In Greek, this is an accounting term that means “to credit to someone’s account.” It was used in contexts where something is assigned or credited to someone that is not intrinsically theirs. 

And with that, Paul's logic is plain to understand. If a person “works,” then the pay that she receives is a matter obligation or fair compensation. But that is not how one is made righteous before God, because righteousness cannot be earned. Instead, it is to be received by grace through faith. Hence, in the middle of verse 5, Paul calls God as the one "who justifies the ungodly."