A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 24:3DELIVERY OF THE LAW AND COVENANT. (Exo. 24:1-18)
Moses came and told the people all the words of the Lord--The rehearsal of the foregoing laws and the ten commandments, together with the promises of special blessings in the event of their obedience, having drawn forth from the people a unanimous declaration of their consent, it was forthwith recorded as the conditions of the national covenant. The next day preparations were made for having it (the covenant) solemnly ratified, by building an altar and twelve pillars; the altar representing God, and the pillars the tribes of Israel--the two parties in this solemn compact--while Moses acted as typical mediator.
exo 24:5young men--priests (Exo 19:22), probably the oldest sons of particular families, who acted under the direction of Moses.
oxen--Other animals, though not mentioned, were offered in sacrifice (Heb 9:18-20).
exo 24:6Moses took half of the blood . . . sprinkled--Preliminary to this was the public reading of the law and the renewed acceptance of the terms by the people; then the sprinkling of the blood was the sign of solemn ratification--half on each party in the transaction.
exo 24:8Moses took the blood, and sprinkled it on the people--probably on the twelve pillars, as representing the people (also the book, Heb 9:19), and the act was accompanied by a public proclamation of its import. It was setting their seal to the covenant (compare Co1 11:25). It must have been a deeply impressive, as well as instructive scene, for it taught the Israelites that the covenant was made with them only through the sprinkling of blood--that the divine acceptance of themselves and services, was only by virtue of an atoning sacrifice, and that even the blessings of the national covenant were promised and secured to them only through grace. The ceremonial, however, had a further and higher significance, as is shown by the apostle (see as above).
exo 24:9Then went up Moses, and Aaron--in obedience to a command given (Exo 24:1-2; also Exo 19:24), previous to the religious engagement of the people, now described.
Nadab, and Abihu--the two oldest sons of Aaron [Exo 6:23].
seventy of the elders--a select number; what was the principle of selection is not said; but they were the chief representatives, the most conspicuous for official rank and station, as well as for their probity and weight of character in their respective tribes.
exo 24:10And they saw the God of Israel--That there was no visible form or representation of the divine nature, we have expressly intimated (Deu 4:15). But a symbol or emblem of His glory was distinctly, and at a distance, displayed before those chosen witnesses. Many think, however, that in this private scene was discovered, amid the luminous blaze, the faint adumbrated form of the humanity of Christ (Eze 1:26; compare Gal 3:24).
sapphire--one of the most valuable and lustrous of the precious gems--of a sky-blue or light azure color and frequently chosen to describe the throne of God (see Eze 1:26; Eze 10:1).
exo 24:11upon the nobles of the children of Israel he laid not his hand--The "nobles," that is, the elders, after the sprinkling of the blood, were not inspired with terror in presence of the calm, benign, radiant symbol of the divine majesty; so different from the terrific exhibitions at the giving of the law. The report of so many competent witnesses would tend to confirm the people's faith in the divine mission of Moses.
eat and drink--feasted on the peace offering--on the remnants of the late sacrifices and libations. This feast had a prophetic bearing, intimating God's dwelling with men.
exo 24:12I will give thee tables of stone--The ten commandments, which had already been spoken, were to be given in a permanent form. Inscribed on stone, for greater durability, by the hand of God Himself, they were thus authenticated and honored above the judicial or ceremonial parts of the law.
exo 24:13Moses went up into the mount of God--He was called to receive the divine transcript. Joshua was taken a little higher, and it would be a great comfort for the leader to have his company during the six days he was in patient waiting for the call on the seventh or sabbath day.
exo 24:14he said unto the elders, Tarry ye here for us--There is a circular valley or hollow a good way up on the brow of Jebel Musa, which was their halting place, while he alone was privileged to ascend the highest peak. The people stood below, as in the "outer court," the elders in the "holy place," Moses, as a type of Christ, in "the holy of holies."
exo 24:18Moses went into the midst of the cloud--the visible token of God's presence. Divine grace animated and supported him to enter with holy boldness.
Moses was in the mount forty days and forty nights--The six days spent in waiting are not included. During that protracted period he was miraculously supported (Deu 9:9), on a peak scarcely thirty paces in compass.