If you really keep the royal law found in Scripture, "Love your neighbor as yourself," you are doing right. But if you show favoritism, you sin and are convicted by the law as lawbreakers. For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it. For he who said, "You shall not commit adultery," also said, "You shall not murder." If you do not commit adultery but do commit murder, you have become a lawbreaker.
Suppose you write a proof that is perfect--except at one point where there is a logical flaw. Does the proof hold? Of course the answer is "no." The proof must have absolutely no logical flaws to be accepted as valid. The same is true of people who try to keep the law of God perfectly. If there is even one small part that is not held perfectly, the entire law is broken. Since we cannot keep the law perfectly, our judgment is death apart from Christ.