Justice, mercy, and faith—or fidelity, faithfulness. The word faith may be somewhat misleading because most tend to think of it in terms of "confidence" or "trust." Jesus uses it in the sense of being faithful, trustworthy, or loyal.
Most people never think of these things in terms of law. Why are they weightier matters of law? The weightiest matter of law is love, and of course, there we cannot forget about sin. Yet justice, mercy, and faithfulness are also matters of law because a person must have a basis—a guide, a standard—from which to judge, to make an evaluation, so he knows whether to be faithful, merciful, or just in his dealing with other people. Why? Because the law is the basis of right and wrong, whether to do or not to do.
People suppose that mercy is not a matter of law. Oh, yes, it is! If the law is done away, there is no basis for mercy—or for kindness or generosity, for that matter. When we must make a judgment, we cannot know whether to be merciful or not unless the law provides us with the foundation.
When we begin to think deeply about these seemingly vague terms, we find that they are directly attached to the law of God. God's law—His instruction, which defines right and wrong—is not contained exclusively in the Ten Commandments. It is not contained just in the five books of the Pentateuch. It is contained in all 66 books; it is everywhere in the Bible.
All of our lives we have been taught, to a greater or lesser extent, to put biblical things into nice, neat, little boxes. We say, "This was for the Jews; that was for the Gentiles. This is for the Christians; that was for Judaism. This is for the Old Testament; that is for the New Testament. This is Old Covenant; that is New Covenant." But Scripture it is not like that!
The Bible was not written like other books. The Old Testament was written primarily with the Christian church and the New Covenant in mind, but it was done within the historical context of ancient Israel. We sometimes have trouble relating to archaic situations, but if we meditate on what troubles us, asking God for guidance, we will see modern applications of the principles contained in the context.
The New Covenant laws, principles, illustrations, and examples are everywhere, intertwined with what we ordinarily think of as "Old Covenant territory." It must be this way because the New Covenant involves the spirit, or intent, of God's law. Sin, righteousness, and love are stated, defined, illustrated, and exemplified throughout the Bible.