Calvin's Commentaries, Vol. 5: Harmony of the Law, Part III, tr. by John King, [1847-50], at sacred-texts.com
18. And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.
18. Et dedit Deus Most, quum finem fecisset loquendi cum eo in monte Sinai, duas tabulas Testimonii, tabulas lapideas scriptas digito Dei.
18 And he gave unto Moses. It must be observed, that, after the voice of God had been heard from the midst of the fire, and He had delivered the Ten Commandments, and the form of the tabernacle had been described, and the work had been already finished by the artificers, though its dedication had not yet taken place, Moses was again withdrawn from the sight and intercourse of men, that he might be taught apart by himself to be a faithful interpreter of the Law. For although God had briefly comprised in the Ten Commandments the sum of His doctrine, which might suffice for the rule of a pious and righteous life, still a clearer exposition was needed, such as Moses afterwards added. With this object he was taken up into the sanctuary (adytum) of heaven, as it were, in order that he might familiarly learn all things that concerned the full and complete understanding of the Ten Commandments, since he could never have attained their genuine meaning if God had not been his Master and Teacher. Hence we gather that he wrote his five books not only under the guidance of the Spirit of God, but as God Himself had suggested them, speaking to him out of His own mouth. Wherefore he observed silence for forty days, that he might afterwards freely speak by the authority of God. Thus ought all true pastors of the Church to be disciples, so as to teach nothing but what they have received. But although God might in a moment have fully perfected His servant, yet, in order more surely to evince that he advanced nothing which did not proceed from the school of heaven, he was separated for forty days from the human race, so that the Israelites might henceforth look up to him as to an angel sent from heaven; for there could be no savour of earth about him who had thus lived with God, without meat and drink, or any other means of nourishment, and divested of all infirmity of the flesh.
Finally, the Ten Commandments were written on two tables, so that they might never be lost. I have elsewhere stated why they were divided into two tables, viz., because they consist of two parts, the first of which is the rule of piety, whilst the second prescribes how we must live righteously, innocently, and chastely with men. Thus the worship of God comes first in order, and then the duties of charity follow. The tables were of stone, inasmuch as it is usual for enduring monuments to be engraven on brass, or stones. That they were “written with the finger of God,” we must understand to mean that the characters were formed without the hand or skill of men, by the secret virtue of God; nor is it a matter of wonder that a writing should have suddenly been brought into existence at the same will (nutu) of God, whereby the waste and shapeless materials of the world, which they call chaos, were changed so as to be resplendent with astonishing elegance and beauty. This expression, however, is metaphorical, whereby what is only applicable to men is figuratively spoken of God; for God is not corporeal so as to write with His finger; and for Him to act is only to command; as it is said in the Psalms,
“He spake, and all things were made; he commanded, and they were created.” (Ps. 33:9, Ps. 148:5.)
Many approve of the allegory, that the Law was written by the Spirit of God on stones, because the hardness of our heart does not receive it without the grace of regeneration; but we must rather hold to the antithesis of Paul, wherein he shews that the Gospel differs from the Law in this respect, because it is written on fleshy hearts, subdued unto obedience, (2Co 3:3;) and indeed it is by no means fitting that we should trifle in such conceits as this, when the simple intention of God is abundantly manifest, viz., that the Law was registered upon stones, in order that the perpetuity of its doctrine should be maintained in all ages.