The sun shall be no more thy light by day. Here Isaiah anticipates one of the most sublime thoughts in the Revelation of St. John the Divine, viz. that the heavenly Jerusalem, illuminated perpetually by the radiance of the Divine Presence, shall need neither light of the sun by day, nor of the moon by night, but shall be sufficiently illumined by the direct and primary light which streams down upon it from God himself.
Whether the sun and moon will continue to exist or not is beyond the prophet's ken—he makes no announcement on the subject; sufficient for him that the redeemed bask perpetually in a Divine radiance shed upon them by the "Father of lights" (see Revelation 21:23; Revelation 22:5).
The germ of the idea appears in the earlier prophecies (Isaiah 24:23). For brightness; rather, for illumination. The Lord shall be unto thee an everlasting light. God is "the Father of lights" (James 1:17)—"the true Light, which lighteth every man that cometh into the world."
(John 1:9). All other light is but his shadow and his reflex—his creature (Genesis 1:3)—therefore perishable, not to be reckoned on for continuance (Psalms 102:26; Hebrews 1:11). But God abides; therefore his light will abide.
He is "the same yesterday, and to-day, and for ever" (Hebrews 13:8). And thy God thy glory (comp. Zechariah 2:5). God will not only be the Light of the Church, but her "Glory" and boast. As the Shechinah was the glory of the first, so "the eternal unchangeable light of Jehovah, with its peaceful gentleness and perfect purity" (Delitzsch), will be the glory of the final temple.