Isaiah 64:1

1.O that thou wouldest rend the heavens! The particle לוא (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. (185) (Genesis 11:5.)

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.” (Psalms 18:11.)

This will appear more plainly from what immediately follows.