Moses reported these words of God to the people, and placed an altar of twelve stones at the foot of the mountain. Then he again ascended the mountain on which the Lord had taken his place, bringing with him Aaron, Nabad, and seventy of the elders. But these were not able to look upon the Lord; nevertheless, they saw the place in which God stood, whose form is related to have been wonderful, and his splendor glorious. Now, Moses, having been called by God, entered the inner cloud which had gathered round about God, and is related to have remained there forty days and forty nights.
During this time, he was taught in the words of God about building the tabernacle and the ark, and about the ritual of sacrifice-things which I, as they were obviously told at great length, have not thought proper to be inserted in such a concise work as the present. But as Moses stayed away a long time, since he spent forty days in the presence of the Lord, the people, despairing of his return, compelled Aaron to construct images. Then, out of metals which had been melted together, there came forth the head of a calf.
The people, unmindful of God, having offered sacrifices to this, and given themselves up to eating and drinking, God, looking upon these things, would in his righteous indignation, have destroyed the wicked people, had he not been entreated by Moses' not to do so. But Moses, on his return, bringing down the two tables of stone which had been written by the hand of God, and seeing the people devoted to luxury and sacrilege, broke the tables, thinking the nation unworthy of having the law of the Lord delivered to them.
He then called around himself the Levites, who had been assailed with many insults, and commanded them to smite the people with drawn swords. In this onset twenty-three thousand men are said to have been slain. Then Moses set up the tabernacle outside the camp; and, as often as he entered it, the pillar of cloud was observed to stand before the door; and God spoke, face to face, with Moses. But when Moses entreated that he might see the Lord in his peculiar majesty, he was answered that the form of God could not be seen by mortal eyes; yet it was allowed to see his back parts; and the tables which Moses had formerly broken were constructed afresh.
And Moses is reported, during this conference with God, to have stayed forty days with the Lord. Moreover, when he descended from the mountain, bringing with him the tables, his face shone with so great brightness, that the people were not able to look upon him. It was arranged, therefore, that when he was to make known to them the commands of God, he covered his face with a veil, and thus spoke to the people in the words of God. In this part of the history an account is given of the tabernacle, and the building of its inner parts. Which having been finished, the cloud descended from above, and so overshadowed the tabernacle that it prevented Moses himself from entering. These are the principal matters contained in the two books of Genesis and Exodus.
Source: Sacred History (New Advent)