The brother of the Lord in James 2:8-11 calls the second greatest commandment “the royal law according to the Scripture”:
Jas. 2:8-11 If you really fulfill the royal law according to the Scripture, “You shall love your neighbor as yourself,” you are doing well. 9 But if you show partiality, you are committing sin and are convicted by the law as transgressors. 10 For whoever keeps the whole law but fails in one point has become accountable for all of it. 11 For he who said, “Do not commit adultery,” also said, “Do not murder.” If you do not commit adultery but do murder, you have become a transgressor of the law.
Here James sees the unity of God’s Law that when one is broken, all are broken. That he is referring to the moral law is seen by his two citations from the Decalogue, commandments six and seven. If we show partiality to our neighbor we are not honoring or loving them and thereby breaking God’s law. We become “transgressor[s] of the law” and by the commandments he cites, the law which he speaks of is the moral law summarized in the Decalogue. See more on this passage, below.
Likewise, the Apostle Paul gives a summary of the second table in Romans 13. We read—
Rom. 13:8-10 Owe no one anything, except to love each other, for the one who loves another has fulfilled the law. 9 For the commandments, “You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,” and any other commandment, are summed up in this word: “You shall love your neighbor as yourself.” 10 Love does no wrong to a neighbor; therefore love is the fulfilling of the law.
To show love to God and to man is the purpose of the Decalogue and that is what it calls us to do. But we may also ask, “How should I love God and my neighbor?” At that time, we look to God’s law to learn how we are supposed to love. See more on this passage, below.
In this passage, Paul cities commandments seven, six, eight and ten and claims that they're summed up in “You shall love your neighbor as yourself”, which is the royal law according to James (Jas. 2:8) and the second greatest commandment according to the Lord Jesus. Paul doesn’t limit himself to these four commandments from the Decalogue, but adds, “any other commandment”. I think that this should be understood in a qualified sense, that is, any other commandment concerning man, not any commandment absolutely. He is here referring to the moral duty toward our fellow man as summarized in the second table of the Decalogue. This is the fulfilling of the law, meaning, in this is the law realized in a person’s life.
Lastly, our Lord in Matthew 19 summarizes the second table in the same way:
Matt. 19:16-22 And behold, a man came up to...