From the early Church Fathers to now.
3 Patristic · 2 Jewish · 1 Catholic
“Nevertheless, I assert according to the Scriptures that He was able to spread out lowly places and open fields, just as He Himself says: 'I will walk before you and make mountains plain.' The force of the water itself could also make deeper things, which had settled, more turbulent: with such a movement of waves and such a surge of a more agitated element, which daily twists the depths of the sea and turns the sands out of the deep.”
“Let us not fear punishment. By this faith we shall escape punishment.… This is the correct attitude of the servants of God to be. For if those who were brought up under the old dispensation, when death was not yet slain, or his "brazen gates broken down" or his "iron bars cut into pieces" so nobly encountered their end, how destitute of all defense or excuse shall we be, if, after having had the benefit of such great grace, we attain not even to the same measure of virtue as they did, now when death is only a name, devoid of reality. For death is nothing more than a sleep, a journey, a migration, a rest, a tranquil haven, an escape from trouble and a freedom from the cares of this present life!”
“By all that is about to be said, he has taught that he has invested Cyrus as king, he has given him the necessary sovereignty to direct his empire with good will and so that the disposition of difficult matters is mitigated and facilitated. This is what the phrase means: "I will level mountains, and I will break to pieces brazen doors."”
“and I will straighten out crooked paths Heb. וַהֲדוּרִים אֲיַשֵּׁר. Jonathan renders: I will level the walls. Comp. (Shab. 77b) “the surrounding area (הדר הדרנא),” since the wall surrounds the city. The word may also be explained as: I will straighten out crooked paths before him.”
“I. The support of God or the heavenly prince of Persia is to be understood; comp. Dan. 10:20. והדורים The crooked places. It is the opposite of ישר straight. Some derive it from הדר glory; it means mountains, and is as to its form either adjective or participle passive. דלתות The doors that are in the gates of the city.”
“Or because of strength, and as to this he says: I will go before you, preparing the place of your victory, by sending terror of you among your adversaries, the great ones, powerful above others, like the Babylonians; the gates of brass, the most powerful cities: he has broken gates of brass, and burst iron bars (Ps 107:16).”
The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.