[James 2:8 KJV] If ye fulfil the royal law according to the scripture, Thou shalt love thy neighbour as thyself, ye do well:
James 2:8 εἰ μέντοι νόμον τελεῖτε βασιλικὸν κατὰ τὴν γραφήν ἀγαπήσεις τὸν πλησίον σου ὡς σεαυτόν καλῶς ποιεῖτε
[Leviticus 19:18 KJV] Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
Matthew 22:36-40 New International Version (NIV)
36 “Teacher, which is the greatest commandment in the Law?” 37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’[a] 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’[b] 40 All the Law and the Prophets hang on these two commandments.”
Leviticus 19:18 refers to this statement "thou shalt love thy neighbour as thyself,"
I believe that James is referring back to Old Testament scripture when he makes this statement, however he is giving it the "Royal" title because it was re-iterated by Jesus who said it was one of the two greatest commandments.
The "Royal Law" of James 2:8 is an allusion to two facts:
- The law alluded to in Lev 19:18 was given by God who was the true King of Israel and also the true King of the Christian community
- It is also an allusion to Peter's well-known designation (1 Peter 2:9) that the Christian community was a continuation of something quite special: "But you are a chosen people, a royal priesthood, a holy nation, God's special possession, that you may declare the praises of him who called you out of darkness into his wonderful light."
This last text is a quote from Ex 19:5, 6 just before God spoke the "Royal Law" and declared:
Although the whole earth is mine, you will be for me a kingdom of priests and a holy nation.
James uses this quotation in a wonderful way. He is discussing the treatment of the poor and needy (James 2:1-7) and how they should be treated - with kindness, respect and great, noble love. He then calls as witness the requirements of the "Royal Law" - as if the quote the (French) aphorism, "noblesse oblige" (Nobility has its obligations).
In other words, James is possibly saying, the royalty of the Church (all its members) acts much better than this and loves those in need. (I agree.)