James 2:8-13

If you really fulfill the royal law according to the Scripture, “You shall love your neighbor as yourself,” you are doing well.  But if you show partiality, you are committing sin and are convicted by the law as transgressors.  For whoever keeps the whole law but fails in one point has become guilty of all of it.  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.  So speak and so act as those who are to be judged under the law of liberty.  For judgment is without mercy to one who has shown no mercy. Mercy triumphs over judgment.

James 2:8-13

Partiality is a sin.  Sinning by the act of partiality causes one to be a transgressor of the law.  This is apparently the opposite of "you shall love your neighbor as yourself".  I guess if you really did love your neighbor as yourself, you wouldn't show partiality toward him or her.

So, how is it that partiality causes transgression of the law?  Well, this is because to fail in one point of the law is to be guilty of all of it.  This still may not add up.  Well, James goes on to explain further:

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.
James 2:11

It's not as if the law can be easily separated.  God is the one who gave the many instructions.  The commands go together and to go against one is to fail to obey what God has said.  In my mind, I am reminded of an AND operator.  God speaks all these commands and they are joined by an AND operator.  If you don't know, AND operator will output a 1 (or True) if all the inputs are 1 (or True).  If any one (or all) are not 1 (or True), then the result will be 0 (or False).

James moves on to tells his readers to act accordingly and points to the law of liberty.  What is the law of liberty?  Or, what is the law of freedom?  I'm not entirely sure, but he is obviously encouraging his readers not to show partiality.  This is as a result of what he has just explained: the sin of partiality is to transgress the law.  What's also key is that James moves on to speak about mercy.  If you don't show mercy, you will not be given mercy.  Perhaps James is pointing his readers towards humility.  We are not in a great situation.  To err in one part of the law is to transgress the whole law.  Yet, believers are free from sin and perhaps that is what James is referring to when he speaks of the "law of liberty".  We should always live knowing that in spite of our situation (lost, depraved, etc), we were shown mercy.  We should show mercy to others.  We shouldn't show favoritism.