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 20:1THE TEN COMMANDMENTS. (Exo. 20:1-26)
And God spake all these words--The Divine Being Himself was the speaker (Deu 5:12, Deu 5:32-33), in tones so loud as to be heard--so distinct as to be intelligible by the whole multitude standing in the valleys below, amid the most appalling phenomena of agitated nature. Had He been simply addressing rational and intelligent creatures, He would have spoken with the still small voice of persuasion and love. But He was speaking to those who were at the same time fallen and sinful creatures, and a corresponding change was required in the manner of God's procedure, in order to give a suitable impression of the character and sanctions of the law revealed from heaven (Rom 11:5-9).
exo 20:2I am the Lord thy God--This is a preface to the ten commandments--the latter clause being specially applicable to the case of the Israelites, while the former brings it home to all mankind; showing that the reasonableness of the law is founded in their eternal relation as creatures to their Creator, and their mutual relations to each other.
exo 20:3Thou shalt have no other gods before me--in My presence, beside, or except Me.
exo 20:4Thou shalt not make . . . any graven image . . . thou shalt not bow down thyself to them--that is, "make in order to bow." Under the auspices of Moses himself, figures of cherubim, brazen serpents, oxen, and many other things in the earth beneath, were made and never condemned. The mere making was no sin--it was the making with the intent to give idolatrous worship.
exo 20:8Remember the sabbath day--implying it was already known, and recognized as a season of sacred rest. The first four commandments [Exo 20:3-11] comprise our duties to God--the other six [Exo 20:12-17] our duties to our fellow men; and as interpreted by Christ, they reach to the government of the heart as well as the lip (Mat 5:17). "If a man do them he shall live in them" [Lev 18:5; Neh 9:29]. But, ah! what an if for frail and fallen man. Whoever rests his hope upon the law stands debtor to it all; and in this view every one would be without hope were not "the LORD OUR RIGHTEOUSNESS" [Jer 23:6; Jer 33:16] (Joh 1:17).
exo 20:18all the people saw the thunderings and the lightnings--They were eye and ear witnesses of the awful emblems of the Deity's descent. But they perceived not the Deity Himself.
exo 20:19let not God speak with us, lest we die, &c.--The phenomena of thunder and lightning had been one of the plagues so fatal to Egypt, and as they heard God speaking to them now, they were apprehensive of instant death also. Even Moses himself, the mediator of the old covenant, did "exceedingly quake and fear" (Heb 12:21). But doubtless God spake what gave him relief--restored him to a frame of mind fit for the ministrations committed to him; and hence immediately after he was enabled to relieve and comfort them with the relief and comfort which he himself had received from God (Co2 1:4).
exo 20:22the Lord said unto Moses--It appears from Deu 4:14-16, that this injunction was a conclusion drawn from the scene on Sinai--that as no similitude of God was displayed then, they should not attempt to make any visible figure or form of Him.
exo 20:24An altar of earth thou shalt make unto me--a regulation applicable to special or temporary occasions.
exo 20:25thou shalt not build it of hewn stone, &c.--that is, carved with figures and ornaments that might lead to superstition.
exo 20:26by steps--a precaution taken for the sake of decency, in consequence of the loose, wide, flowing garments of the priests.