Thomas Aquinas

He foretells the fulfilment: every valley, following the metaphor of the evil way: if it is mountainous, it causes labor, and thus is said, every valley shall be exalted, so that it is made as high as the mountains and everything may be made level; if it is winding, it causes wandering, and as to this he says, the crooked shall become straight; if it is rocky, it causes pain in the feet, and as to this: the rough ways plain; through which is signified that faintheartedness will be turned into security, pride into humility, wickedness …

Commentary on Isaiah ↗