Isaiah 35:3

Then let us flee the wickedness of this world, in which "the very days are evil," and flee it relentlessly. On that account Isaiah cries out, "Be strong, you hands which are feeble and you knees which are without strength." This means: Be strong, you knees, not of the body but of the soul, so that the footstep of the spirit can rise up straightway to the heights of heaven. Thus conduct will be more stable, life more mature, grace more abundant and discretion more guarded.

Now we have this prophecy fulfilled in the Gospels, partly, when they brought to our Lord and Savior a paralytic lying on a bed, who he made whole with a word; and partly, when many that were blind and possessed with demons, yes, laboring under various diseases and weaknesses, were released from their sufferings by his saving power. Nor should we forget how even now throughout the whole world multitudes bound by all forms of evil, full of ignorance of Almighty God in their souls, are healed and cured miraculously and beyond all argument by the medicine of his teaching. Except that now we call him God as we should, as one who can work thus, as I have already shown in the evidence of his divinity. Yes, surely it is right now to acknowledge him to be God, since he has given proof of power divine and truly inspired.For it was specifically God's work to give strength to the paralyzed, to give life to the dead, to supply health to the sick, to open the eyes of the blind, to restore the lame and to make the tongue-tied speak plainly, all of which things were done by our Savior Jesus Christ, because he was God. And they have been witnessed to by many throughout all the world that preach him—whose evidence unvarnished and truthful is confirmed by trial of torture, and by persistence even to death, which they have shown forth before kings and rulers and all nations, witnessing to the truth of what they preach.

The grace of this time in which John was exhorting sinners to repentance and baptizing those who confessed their sins in the desert, Isaiah previously witnessed when he said, "The desert will rejoice and blossom like the lily. The desert of the Jordan will bloom …

The sick of the palsy is healed, and that in public, in the sight of the people. For, says Isaiah, "they shall see the glory of the Lord and the excellence of our God." What glory, and what excellence? "Be strong, you weak hands and feeble knees" refers to the palsy. "Be strong; fear not." "Be strong" is not vainly repeated, nor is "fear not" vainly added; because with the renewal of the limbs there was to be, according to the promise, a restoration also of bodily energies: "Arise, and take up your couch"; and likewise moral courage not to be afraid of those who should say, "Who can forgive sins, but God alone?"

(Verse 3, 4.) Strengthen the weak hands, and make firm the feeble knees. Say to those who are fearful, "Be strong, do not fear! Behold, your God will bring vengeance and retribution. God Himself will come and save you (or as the Septuagint translates, us). Then the eyes of the blind will be opened, and the ears of the deaf will hear. Then the lame will leap like a deer, and the tongue of the mute will sing, for water will burst forth in the wilderness, and streams in the desert, and the dry ground will become a pool (or marsh), and the thirsty land springs of water. In the dens where the dragons once lived, green rushes and reeds shall grow (in the version of the Seventy: There will be the joy of birds and the folds of flocks). And there shall be a path and a road (or a clean road) and it shall be called the holy road, no unclean person shall pass through it. And this shall be for us a direct road, so that fools shall not go astray (or as the Seventy translated, There will be no unclean road there, those who are scattered shall pass through it and shall not wander. There will be, it says, no lion there, and no wicked beast shall ascend it, nor shall it be found there; but they who are set free shall walk on it. And those who are redeemed by the Lord shall return and come to Zion with praise, and everlasting joy shall be upon their heads. They shall obtain joy and gladness, and sorrow and sighing shall flee away. […]