Actually you are mixing a New Testament Command given by James with a truth revealed by Jesus.
James said,
"For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all." (James 2:10).
Jesus said,
"Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven." (Matthew 5:19).
In James 2:10, James is talking about the Royal Law or the Law of love under the New Covenant (For it is called the Royal Law in verse 8 and the Law of Liberty in verse 12; So this is not talking about the Law of Moses). In Matthew chapter 5, Jesus does give us many Preparational New Testament Commandments that are repeated later in the New Testament (After His death); However, in Matthew 5:19, Jesus is talking exclusively about the Law of Moses (See verses 17-18). Also, see verse 24. For it is a Command that is specifically only for the Old Covenant because Jesus says to reconcile with your brother before giving a gift (animal sacrifice) at the altar. Christians do not sacrifice animals anymore.