First, man needs the incomparable God (vv. 18-26). Fading man needs the eternal God, the only one who does not wither and fade but abides forever.
Second, man needs the coming Christ, who is to be announced as the glad tidings. The coming Christ is to be announced as Jehovah our God (v. 3) and as Jehovah of glory, to be revealed and seen by all flesh together (v. 5). Furthermore, the coming Christ is to be announced as the Lord Jehovah coming with might to rule with His arm, having His reward with Him and His recompense before Him (vv. 9-10). Finally, the coming Christ is to be announced as a Shepherd feeding His flock, gathering the lambs in His arms, carrying them in His bosom, and leading those who are nourishing the young (v. 11).
Verses 6 through 8 indicate that man also needs regeneration with the living and abiding word of God (1 Pet. 1:23). God is invisible, abstract, and mysterious, but He is embodied in His word. Now by touching the word as the embodiment of God, we receive for our regeneration a word that is living and abides forever.
Finally, according to Isaiah 40, man needs to wait on the eternal God, Jehovah. To wait on God is to be terminated and replaced by the Triune God. Concerning Him, verse 28 says, "Do you not know, / Or have you not heard, / That the eternal God, Jehovah, / The Creator of the ends of the earth, / Does not faint and does not become weary. / There is no searching of His understanding." His understanding is not searchable.
As the eternal God, Jehovah empowers those who are weary. "He gives power to the faint, / And to those who have no vigor He multiplies strength. / Although youths will faint and become weary, / And young men shall collapse exhausted; / Yet those who wait on Jehovah will renew their strength; / They will mount up with wings like eagles; / They will run and will not faint / They will walk and will not become weary" (vv. 29-31). The wings here signify the resurrection power of Christ. Those who stop themselves and wait on Jehovah will experience this resurrection power, be transformed, and soar in the heavens.
When we wait on the eternal God, we are terminated and replaced by Him, and then we have Him as our life and power, which is the power of resurrection. This power strengthens us and enables us to mount up with wings as eagles and to soar above the earth. This is the full experience of God's salvation revealed in chapter forty of Isaiah.