O that thou wouldest rend the heavens! The particle lv' (lu) appears to me, in this passage, to denote a wish; for, although it has many significations, yet the context shews that this signification is more appropriate to this passage than any other. Here believers burst forth into earnest prayer, as usually happens, when in sore adversity we do not find plain terms to be sufficiently forcible for our purpose. God is said to "rend the heavens," when he unexpectedly gives some uncommon and striking proof of his power; and the reason of this mode of expression is, not only that men, when they are hard pressed, commonly look up to heaven, from which they expect assistance, but that miracles, by interrupting the order of nature, open up for themselves an unusual path. Now, when God renders no assistance, he appears to be shut up in heaven, and to disregard what is taking place on earth. For this reason he is said to open and "rend the heavens," when he holds out to us some testimony of his presence; because otherwise we think that he is at a great distance from us. That thou wouldest come down. This expression, like the former, is adapted to the estimation of our flesh; for God does not need to move from one place to another, but accommodates himself to us, that we may understand those subjects better. [1] (Genesis 11:5; 18:21.) Let the mountains flow down. That is, "Let thy majesty be openly displayed, and let the elements, struck by the perception of it, yield and obey." (Psalm 18:11.) This will appear more plainly from what immediately follows.

Footnotes

1 - "Afin que nous comprenions mieux ce qui nons est dit de luy." "That we may understand better what is said to us about him."

Oh that thou wouldest rend the heavens - That is, in view of the considerations urged in the previous chapter. In view of the fact that the temple is burned up Isaiah 64:11; that the city is desolate; that the land lies waste, and that thine own people are carried captive to a distant land. The phrase 'rend the heavens,' implies a sudden and sublime descent of Yahweh to execute vengeance on his foes, as if his heart was full of vengeance, and the firmament were violently rent asunder at his sudden appearance. It is language properly expressive of a purpose to execute wrath on his foes, rather than to confer blessings on his people. The latter is more appropriately expressed by the heavens being gently opened to make way for the descending blessings. The word rendered here 'rend' (קרע qâra‛), means properly to tear asunder, as, e. g., the garments in grief Genesis 37:29; 2-Samuel 13:31; or as a wild beast does the breast of anyone Hosea 13:8. The Septuagint, however, render it by a milder word - ἀνοίξης anoixēs - 'If thou wouldst open the heavens,' etc. So the Syriac renders it by 'O that thou wouldst open,' using a word that is usually applied to the opening of a door. God is often represented as coming down from heaven in a sublime manner amidst tempests, fire, and storms, to take vengeance on his foes. Thus Psalm 18:9 :
He bowed the heavens also and came down;
And darkness was under his feet.
Compare Habakkuk 3:5-6. It should be remembered that the main idea in the passage before us is that of Yahweh coming down to destroy his foes. His people entreat him to descend with the proofs of his indignation, so that every obstacle shall be destroyed before him, Thus he is described in Psalm 144:5-6 :
Bow thy heavens, O Lord, and come down;
Touch the mountains, and they shall smoke;
Cast forth lightning, and scatter them,
Shoot out thine arrows, and destroy them.
That the mountains might flow down at thy presence - The idea here is, that the presence of Yahweh would be like an intense burning heat, so that the mountains would melt and flow away. It is a most sublime description of his majesty, and is one that is several times employed in the Bible. Thus in relation to his appearance on Mount Sinai, in the song of Deborah Judges 5:4-5 :
The earth trembled and the heavens dropped,
The clouds also dropped water.
The mountains melted from before Yahweh,
Even Sinai from before Yahweh, the God of Israel.
So Psalm 97:5 :
The hills melted like wax at the presence of Yahweh,
At the presence of Yahweh (the God) of the whole earth.
So also in Micah 1:3-4 :
Lo, Yahweh cometh forth out of his place,
And will come down and tread upon the high places of the earth
And the mountains shall be molten under him.
And the valleys shall be cleft,
As wax before the fire,
And as the waters pour down a precipice.

O that thou wouldest rend the heavens - This seems to allude to the wonderful manifestation of God upon Mount Sinai.

O that thou wouldest (a) rend the heavens, that thou wouldest come down, that the mountains might flow down at thy presence,
(a) The prophet continues his prayer, desiring God to declare his love toward his Church by miracles and mighty power, as he did in mount Sinai.

O that thou wouldst rend the heavens, that thou wouldst come down,.... Before, the church prayed that the Lord would look down from heaven and behold, Isaiah 63:15, now that he would open the heavens, and descend from thence; not by change of place, for he fills heaven and earth with his presence; but by some visible display of his power, in destroying her enemies, and delivering her from them. Some take this to be a prayer for the first coming of Christ from heaven to earth, by his incarnation, in order to redeem and save his people; and others that it is for his second coming to judgment, to take vengeance on his adversaries, when his wrath will burn like fire; but rather it is for his spiritual coming, to avenge his church and people on antichrist, and the antichristian states. She had seen him, as a triumphant conqueror, stained with the blood of his enemies; and now she prays for the accomplishment of what she had seen in vision and prophecy:
that the mountains might flow down at thy presence; kings and princes of the earth, and kingdoms and states governed by them, compared to mountains for their seeming firmness and stability; yet these will melt like wax, and flow like water, tremble and disappear at the presence of the King of kings, when he comes forth in his great wrath against them; as it is explained in the next verse,
that the nations may tremble at thy presence; see Revelation 16:20. Here ends the sixty third chapter in the Targum.

They desire that God would manifest himself to them and for them, so that all may see it. This is applicable to the second coming of Christ, when the Lord himself shall descend from heaven. They plead what God had used to do, and had declared his gracious purpose to do, for his people. They need not fear being disappointed of it, for it is sure; or disappointed in it, for it is sufficient. The happiness of his people is bound up in what God has designed for them, and is preparing for them, and preparing them for; what he has done or will do. Can we believe this, and then think any thing too great to expect from his truth, power, and love? It is spiritual and cannot be comprehended by human understanding. It is ever ready. See what communion there is between a gracious God and a gracious soul. We must make conscience of doing our duty in every thing the Lord our God requires. Thou meetest him; this speaks his freeness and forwardness in doing them good. Though God has been angry with us for our sins, and justly, yet his anger has soon ended; but in his favour is life, which goes on and continues, and on that we depend for our salvation.

TRANSITION FROM COMPLAINT TO PRAYER. (Isaiah 64:1-12)
rend . . . heavens--bursting forth to execute vengeance, suddenly descending on Thy people's foe (Psalm 18:9; Psalm 144:5; Habakkuk 3:5-6).
flow down-- (Judges 5:5; Micah 1:4).

The similes which follow cannot be attached to this nâzōllū, however we may explain it. Yet Isaiah 64:1 (2) does not form a new and independent sentence; but we must in thought repeat the word upon which the principal emphasis rests in Isaiah 63:19 (Isaiah 64:1). "(Wouldst come down) as fire kindles brushwood, fire causes water to boil; to make known Thy name to Thine adversaries, that the heathen may tremble before Thy face! When Thou doest terrible things which we hoped not for; wouldst come down, (and) mountains shake before Thy countenance!" The older expositors gave themselves a great deal of trouble in the attempt to trace hămâsı̄m to mâsas, to melt. But since Louis de Dieu and Albert Schultens have followed Saadia and Abulwlid in citing the Arabic hms, to crack, to mutter, to mumble, etc., and hšm, to break in pieces, confringere, from which comes hashim, broken, dry wood, it is generally admitted that hămâsim is from hemes (lit. crackling, rattling, Arab. hams), and signifies "dry twigs," arida sarmenta. The second simile might be rendered, "as water bubbles up in the fire;" and in that case mayim would be treated as a feminine (according to the rule in Ges. 146, 3), in support of which Job 14:19 may be adduced as an unquestionable example (although in other cases it is masculine), and אשׁ = בּאשׁ would be used in a local sense, like lehâbhâh, into flames, in Isaiah 5:24. But it is much more natural to take אשׁ, which is just as often a feminine as מים is a masculine, as the subject of תּבעה, and to give to the verb בּעה, which is originally intransitive, judging from the Arabic bgâ, to swell, the Chald. בּוּע, to spring up (compare אבעבּעות, blisters, pustules), the Syr. בּגא, to bubble up, etc., the transitive meaning to cause to boil or bubble up, rather than the intransitive to boil (comp. Isaiah 30:13, נבעה, swollen = bent forwards, as it were protumidus). Jehovah is to come down with the same irresistible force which fire exerts upon brushwood or water, when it sets the former in flames and makes the latter boil; in order that by such a display of might He may make His name known (viz., the name thus judicially revealing itself, hence "in fire," Isaiah 30:27; Isaiah 66:15) to His adversaries, and that nations (viz., those that are idolaters) may tremble before Him (מפּניך: cf., Psalm 68:2-3). The infinitive clause denoting the purpose, like that indicating the comparison, passes into the finite (cf., Isaiah 10:2; Isaiah 13:9; Isaiah 14:25). Modern commentators for the most part now regard the optative lū' (O that) as extending to Isaiah 64:2 also; and, in fact, although this continued influence of lū' appears to overstep the bounds of the possible, we are forced to resort to this extremity. Isaiah 64:2 cannot contain a historical retrospect: the word "formerly" would be introduced if it did, and the order of the words would be a different one. Again, we cannot assume that נזלּוּ הרים מפּניך ירדתּ contains an expression of confidence, or that the prefects indicate certainty. Neither the context, the foregoing נוראות בּעשׂותך נו (why not עשׂה?), nor the parenthetical assertion נקוּה לא, permits of this. On the other hand, וגו בעשׂותך connects itself very appropriately with the purposes indicated in Isaiah 64:1 (2.): "may tremble when Thou doest terrible things, which we, i.e., such as we, do not look for," i.e., which surpass our expectations. And now nothing remains but to recognise the resumption of Isaiah 63:19 (Isaiah 64:1) in the clause "The mountains shake at Thy presence," in which case Isaiah 63:19b-64:2 (Isaiah 64:1-3) forms a grand period rounded off palindromically after Isaiah's peculiar style.

Rent - A metaphor taken from men, that when they would resolutely help one in distress, break and fling open doors and whatever may hinder. Flow down - That all impediments might be removed out of the way: possibly an allusion to God's coming down upon mount Sinai, in those terrible flames of fire.