As noted in a recent class discussion, Christians dedicated to living by every word that proceeds from the mouth of God (Mt. 4:4), are derided for trying to keep the letter of the law, when their efforts rather should be expended on keeping the spirit of the law, whatever that is. On the face of it, that is patently ridiculous: God gives us commands to obey in ordering our lives and service and then says, "You don't really have to obey them, just keep the spirit or tenor of them!"
Given that the words "spirit," "law," and "letter" are found in the New Testament, if the above construct criticizing devoted obedience is not valid, there must be a disconnect between how those words are used in scripture and how some apply them. To begin with, they are wrongly assembled. There is no where in the entire original text where you will find the expressions, "Spirit of the law," or "Letter of the Law," (though regarding the latter, "of the law" does appear in italics as an interpolation in one verse in the NASV). But, nowhere does God's word propose keeping the spirit of the law. You read right. Read the sentenced again.
Here is what the Bible says: "...God...made us...ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life" (II Cor. 3:4); "We have been discharged from the law, having died to that wherein we were held; so that we serve in newness of the spirit, and not in oldness of the letter" (Romans 7:6); and of God's "spiritual Jew" under the new covenant, "He is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh. But he is a Jew who is one inwardly, and circumcision is that of the heart, in the spirit, not in the letter" (Rom. 2:28-29).
So what is meant by letter and spirit? In the first text, new covenant and letter are contrasted. In explanation, spirit and letter are contrasted, thus spirit is equated with the new covenant. The letter is represented by that engraved on stones, thus the law of Moses (II Cor. 3:6-7). The term spirit referring to the new covenant, requires a new condition of covenant participation. Thus the second passage (Rom. 7:6), says we have been discharged from mere law, the "letter," and serve in newness of spirit. What this new requirement is we shall see momentarily.
If "the letter" refers to the law of Moses as well as any system of mere law, which leaves us in condemnation, what is the contrast? For if "the spirit" references the new covenant, seeing that there are laws therein, and it is written as well as was the old covenant, what is the difference? That becomes clear in our third text (Rom 2:28-29). Under the first covenant, the letter, one was a Jew who was born in and of Israel. The law, the letter, decreed all Israel to be associates of the covenant. Thus King Saul was of Israel and a covenant object though spiritually flawed. The lad Samuel was a subject of the covenant, and "ministered before the Lord" with Eli even before he knew the Lord (I Sam. 3:1, 7). By being of Israel they had covenant status by decree, by letter. But now we must know the Lord to participate in the new covenant (Heb. 8:11). Now it is our heart that is to be circumcised. I am to be a child of God in my heart, in the spirit. Thus one's relation to God is not because he is a Jew, established by letter, or law of the old covenant, but individually each must be a child of God in his spirit. He must take God's laws into his mind, and have them written on his heart (Heb. 8:10). New covenant relation depends upon the nature of his individual spirit, not upon a decree or letter covering all in an outward physical relationship, a difference in old Israel and new Israel, the church. Or as Robert Turner once put it, one does not belong to Christ because he attends the services of a certain church, but he attends such because he belongs to Christ. To mitigate obeying God's law fails to see that his laws are so important I must take them into myself, into my very heart, mind, and spirit.
As a closing implication, under the new order there is no decree, or letter, that by birth makes one part of a covenant. For even those who had such status may have wished to break its laws. But with God's law in one's spirit his greatest desire is to keep it. He is a child of God in spirit.