A generation or two ago it would be common to hear people ask something along the lines of, "If you were to die today and met Jesus at the heavenly gates, if He asked you, 'Why should I let you into my heaven?' What would you say?" While that's definitely not my favorite question, I do understand the intention. The motivation of that or similarly shaped questions is to help a person determine if they know the gospel, the good news of Jesus. Romans 4 is rooted in this exact same motivation. Paul wants both the Jewish and Gentile background believers to know how to be made right with God.