For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it.This verse is a great teaching verse when it comes to understanding that we are sinners, and that we are guilty.
When we sin, whether it is a big sin or a small sin (speaking in human terms...) we have broken God's law, making us law breakers. There are no big sinners or small sinners in God's worldview. There are simply sinners, and they are all in need of a savior. It is for this reason Jesus came. He lived, without sin, without becoming a law breaker, then took our sins upon Himself, was crucified, died, and was buried. Then on the third day he rose again!
As law breakers, let us be thankful that a savior came, because there is not a one of us who is not a law breaker.
Notice in the previous verse, that showing partiality is breaking God's law.