notwithstanding that they had been wont [to enjoy Our blessings and] to hew out dwellings from the mountains, [wherein they could live] in security

Out of the mountains did they hew (their) edifices, (feeling themselves) secure.

And they used to hew out dwellings from the hills, (wherein they dwelt) secure.

And they hewed houses in the mountains, in security.

And they used to carve from the mountains, houses, feeling secure.