John Calvin on Isaiah 40:29

29. He giveth power to the faint. The Prophet now applies to the present subject the general statements which he made; for we have said that his intention was to give warmer encouragement to the people, and to lead them to cherish better hope. Because the Jews were at that time weakened and destitute of all strength, he shews that on this account it belongs to God to give assistance to those who were thus exhausted and weakened. He therefore magnifies the power of God on this ground, that they may conclude and believe that they ought not to doubt of their salvation so long as they enjoy his favor. It was indeed to the people who were held captive in Babylon that the Prophet looked; but we ought also to apply this doctrine to ourselves, that whenever our strength shall fail, and we shall be almost laid low, we may call to remembrance that the Lord stretches out his hand ‘to the faint,” who are sinking through the want of all help. But first, we must feel our faintness and poverty, that the saying of Paul, “The power of God is made perfect in our weakness,” (2 Corinthians 12:9,) may be fulfilled; for if our hearts are not deeply moved by a conviction of our weakness, we cannot receive seasonable assistance from God.