John Calvin on Exodus 24:4

4. And Moses “Had written.” — Lat. wrote all the words of the Lord. This parenthesis is opportunely inserted; for we shall see a little further on that the book was read before the people; but, in order to awaken greater attention, before the reading he built an altar and offered victims in the sight of all the people. Moreover, it must be observed that statues “Pillars.” — A V. “Some think that this altar was set upon twelve stones, such as Elias built, 1 Kings 18:31; and Joshua 4:20; in which places, however, the word used is אבנים, (abanim,) which signifieth stones, which were gathered together to make one altar or heap; but here the word is מצבה, (matsebah,) which is a pillar, so called a stando, because it standeth alone, and is erected and set up as a monument.” — Abridged from Willet in loco. See ante, vol. 2, p. 117, and note were erected near the altar according to the number of the tribes, that they might know that they were not kept afar off in token of rejection, but only that, conscious of their own unworthiness, they might humble themselves before God in fear and trembling; for, though they were removed to a considerable distance, still they were remembered before God, and thus He embraced them all, as it were, by means of these statues. What Moses, however, calls by this name, were not images bearing the shape of a man, but heaps of stones, which might be as monuments representing the twelve tribes; that they might know that they were by no means excluded from the sanctity of the altar.