2 Ye are our epistle written in our hearts, known and read of all men:
3 Forasmuch as ye are manifestly declared to be the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in fleshy tables of the heart.
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Speaking this way, Paul was certainly remembering the words of the prophet Jeremiah, who said that the new, messianic covenant would become possible only when the Torah was not only carved on stone, but also written in each heart. Speaking of the 'letter of Christ,' the apostle has in mind above all this inner Torah. In Gospel times people spoke much and often about the inner Torah; in the consciousness of a believing Jew of that era the path of the inner Torah was inseparable from ideas of righteousness. The ideal of righteousness, an unattainable ideal, was a person who had become a living Torah, a person whose life would be determined only by the inner Torah and by nothing else.
This ideal was unattainable because it was impossible for fallen humanity, whose nature had been damaged by the fall, to become a living Torah: for that one had to become sinless, to outlive completely in oneself the consequences of the fall. The letter of the Torah pointed a person to a path that fallen humanity could not walk. A person could only die on this path, being freed from the power of sin at the moment of death; but to remain alive and become a living Torah was unreal for fallen humanity. The Torah carved on stone was a witness against the person, and service to it was a ministry of condemnation.
The more a person strove toward the ideal, toward having the inner Torah determine his whole life, toward becoming a living Torah, the more he became convinced that the goal was unattainable. The closer the goal appeared, the stronger became the barrier of sin separating the one walking from it. In the end the goal turned out to be very near, and at the same time absolutely unattainable.
Only the breath of God, the breath of the Kingdom, which entered the world through Christ, could change the situation radically. The coming of Christ made what had previously been impossible possible: the living Torah ceased to be an unattainable ideal of the righteous person and became a fully real goal of his path. More than that: otherwise one could not enter the Kingdom. There, in the Kingdom, there could be no place for any sin by definition.
Therefore not only the One who brought the Kingdom into the world had to be free from sin, but also everyone who wanted to become its inhabitant. That is why the first Christians looked at Christ as the living Torah, themselves striving with His help toward that same goal. In this sense every Christian truly was a 'letter of Christ': only the One who Himself manifests the living Torah can make into a living Torah the one who trusted Him and followed Him. Thus the 'letter of Christ,' written in the hearts of the faithful, becomes for them a pass into the Kingdom and the path of salvation.