John Calvin Commentary Isaiah 64:1

"Oh that thou wouldest rend the heavens, that thou wouldest come down, that the mountains might quake at thy presence," — Isaiah 64:1 (ASV)

Oh, that you would rend the heavens! The particle לוא (lu) appears to me, in this passage, to denote a wish. For, although it has many significations, the context shows that this signification is more appropriate to this passage than any other. Here believers burst forth into earnest prayer, as usually happens when, in severe 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. The reason for this mode of expression is not only that people, when they are hard-pressed, commonly look up to heaven from which they expect assistance, but also that miracles, by interrupting the order of nature, open up an unusual path for themselves.

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 you would come down. This expression, like the former, is adapted to our human way of thinking; for God does not need to move from one place to another, but accommodates himself to us so that we may understand these subjects better (Genesis 11:5; Genesis 18:21).

Let the mountains flow down. That is,

“Let your majesty be openly displayed, and let the elements, struck by the perception of it, yield and obey” (Psalms 18:11).

This will appear more clearly from what immediately follows.