Exodus 31:18 reads;
When the LORD finished speaking to Moses on Mount Sinai, He gave him the two tablets of the Testimony, the tablets of stone inscribed by the Finger of God.
The Law had been engraved on stone by the hand of God himself; and had thus the strongest proofs of divine origin, and the divine attestation to its pure and holy nature.
When Moses approached the camp and saw the calf and the dancing, his anger burned and he threw the tablets out of his hands, breaking them to pieces at the foot of the mountain (Exodus 32:19).
They were frail, and easily broken. There was no life in them; He has made us competent as ministers of a new covenant--not of the letter but of the Spirit; for the letter kills, but the Spirit gives life 2 Corinthians 3:6) and valuable and important as they were.
But the Spirit of the living God in strong contrast with letters engraved on stone is moving on the heart, and producing that variety of graces which constitute so striking and so beautiful an evidence of your conversion. If written by the Spirit of the living God, it was far more valuable, and precious, and permanent than any record which could be made by ink.
But in fleshly tables of the heart - In truths engraved on the heart. This testimonial was of more value than an inscription on stone, because:
(1) No hand but that of God could reach the heart, and inscribe these truths there.
(2) because it would be attended with a life-giving and living influence. It was not a mere dead letter.
(3) because it would be permanent. Stones, even where laws were engraved by the finger of God, would molder and decay, and the inscription made there would be destroyed. But not so with that which was made on the heart. It would live forever.