Thou shalt not build it of hewn stone - Because they were now in a wandering state, and had as yet no fixed residence; and therefore no time should be wasted to rear costly altars, which could not be transported with them, and which they must soon leave. Besides, they must not lavish skill or expense on the construction of an altar; the altar of itself, whether costly or mean, was nothing in the worship; it was only the place on which the victim should be laid, and their mind must be attentively fixed on that God to whom the sacrifice was offered, and on the sacrifice itself, as that appointed by the Lord to make an atonement for their sins.

And if thou wilt make me an altar of stone,.... If they chose instead of an earthen one to make one of stone, as they might in rocky places, where they came, and in such an one where they now were, Mount Sinai, under which hill an altar was built, Exodus 24:4,
thou shall not build it of hewn stone; which would require time and occasion expense, to hew and polish them in an artificial way; but it was to be built of rude and unpolished stones, just as they were taken out of the quarry, or found lying by the way, and which were laid up in an heap one upon another, and was done with little trouble, and without any ornament, and easily separated and thrown down, when become useless: the reason of this law, as given by Maimonides (w), is this,"because the idolaters of that time built their altars of hewn stones, therefore God forbad it, lest we should be like them, and that we might shun it in all things, he commanded the altar to be made of earth, as it is said, an altar of earth shalt thou make unto me; and if it could not be made without stones, that the stones should remain in their own natural form, and be neither hewn nor polished; as he after forbad a painted stone, and a plantation of trees by an altar; for in each of these there is one and the same intention and design, namely, that we might not worship him in the same manner in which idolaters used to worship their fictitious deities:"
for if thou lift up thy tool upon it; or, thy sword (x); it signifies any tool or instrument made of iron as a sword is, and here such an one as is used in hewing of stone; which, if lifted up on the altar, or on any of the stones of which it is built, to strike and hew them with:
thou hast polluted it; and so made it unfit for use: how this should be done hereby is not easy to say, no good reason seems to be assignable for it but the will and pleasure of God; who so appointed it, and reckoned that a pollution, and would have it so thought by others, which with men is accounted ornamental; his thoughts and judgment are not as man's: the Targum of Jonathan is,"for if thou liftest up iron, of which a sword is made, upon a stone, thou wilt profane it;''the reason which the Misnic doctors (y) give, and Jarchi from them, is,"because iron was created to shorten the days of men, but the other was made to prolong the days of men: and therefore it cannot be just that that which shortens should be lifted up and agitated over that which prolongs:''but Maimonides gives a better reason of it, as Abarbinel understands him, which was to prevent persons making images in stones (z), which image making is the thing guarded against and forbidden in the context; but still better is that of Isaac Arama (a), that the hands of the artificer were to abstain from the stones of the altar, lest that good which men obtain of God at the altar should be attributed to any work of theirs: though, after all, it is right what Aben Ezra, says, that it does not belong to us to search after the reasons of the commands, at least not in too curious and bold a manner, and where God is silent and has not thought fit to give any.
(w) Moreh Nevochim, par. 3. c. 45. (x) "gladium tuum", Montanus, Piscator, Cartwright. (y) Misnah Middot, c. 3. sect. 4. (z) Apud L'Empereur in Middot, ib. (a) Apud Rivet in loc.

thou shalt not build it of hewn stone, &c.--that is, carved with figures and ornaments that might lead to superstition.