Isaiah 63:13 - Matthew Poole's English Annotations on the Holy Bible

That led them through the deep; showing that God did not dry up shallow places, but the very depth of the sea, the very channel, which is the deepest part. Or, between those heaps of waters that stood up as a wall on each side of them, which might make it seem terrible, and therefore it is ascribed to their faith, Hebrews 11:29. As an horse in the wilderness; or, plain; for so wilderness is sometimes taken, and may be here meant, by comparing it to a valley in the next verse, viz. with as much safety as the horse runs up and down in the plain ground; or, with as much ease and tenderness as a horse led by the bridle; not as men affrighted, but soberly and orderly. That they should not stumble: this may be taken metaphorically, they came to no harm; or properly, that though the sea were but newly divided, yet it was so dried, that the mud, as also the unevenness of the ground, was not any occasion of their stumbling, or their sticking in it; probably so dried and smoothed by the wind that God sent as it were to prepare the way before them. See Isaiah 40:3-5.