8 Yes indeed, it is good when you obey the royal law as found in the Scriptures: “Love your neighbor as yourself.” 9 But if you favor some people over others, you are committing a sin. You are guilty of breaking the law. 10 For the person who keeps all of the laws except one is as guilty as a person who has broken all of God’s laws. 11 For the same God who said, “You must not commit adultery,” also said, “You must not murder.” So if you murder someone but do not commit adultery, you have still broken the law.
“Rabbi Akiva (lived ca. A.D. 50-135), a beloved figure in Jewish history, said that Leviticus 19:18 is “a great principle… in the Torah.” James’s contribution was to call Leviticus 19:18 “the royal law” (James 2:8), something no other Bible writer did. Why did James use this term?
First, James used the term “royal” because Jesus, the true King, taught that Leviticus 19:18 summed all relationships with people. As with Jesus (cf. Matthew 22:39; Romans 13:9; Galatians 5:14), for James neighbor love along with love for God summed up and fulfilled the whole law (cf. v. 9). Second, James believed this law to be “royal” because of its effects on our lives. It helps us become mature and complete (James 1:4). It provides a sample, or “firstfruits,” of what the power of God’s love can do in a person’s life (v. 18). It fulfills the righteous life that God requires (v. 20).”
Biblical Illustrator Treasury – James, (Nashville, TN: LifeWay Christian Resources, 2017)