James 2:10-13 For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all. For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law. So speak you, and so do, as they that shall be judged by the law of liberty. For he shall have judgment without mercy, that hath showed no mercy; and mercy rejoice against judgment.
Guilty of all means that one sin is enough to damn the soul (Rom. 5:12-21). The obligation to law is total (Deut. 27:26; Gal. 3:10). This applies to all laws that have the death penalty (Rom. 1:32; 1Cor. 6:9-11; Gal. 5:19-21). One is guilty of lawbreaking whether he breaks one or many laws. The authority which gave one commandment gave all of them and the one who resists Him by breaking one law is guilty of the whole. Do not commit adultery … Do not kill: These commandments are in both covenants, the old and the new (Exo. 20:13-14; Mat. 5:21-32; Rom. 13:9). We are not now under obligation to obey them because they are part of the old covenant which is abolished, but because they are part of the new.
The law of liberty refers to the new covenant, not the old. It is by the gospel, men of this age will be judged (Rom.2:16). The ones who receive mercy and show mercy rejoice in the fact that they will not face judgment because of obeying the law. They will be exalted by mercy above judgment. There is no mercy in law. The meaning is that mercy through grace will triumph over law because the demands of the law have been met by grace and the lawbreakers are justified by it through faith to escape the judgment of the law (John 3:36; 5:24) until such a person sin willfully after he has received the knowledge of the truth, there remains no more sacrifice for sins (Heb. 10:26).