A.T. Robertson Commentary Revelation 21:23

"And the city hath no need of the sun, neither of the moon, to shine upon it: for the glory of God did lighten it, and the lamp thereof [is] the Lamb." — Revelation 21:23 (ASV)

To shine upon it (ινα φαινωσιν αυτη). Purpose clause with ινα and the present active subjunctive of φαινω, to keep on shining. Light is always a problem in our cities. See Isa 60:19f..

Did lighten it (εφωτισεν αυτην). First aorist active indicative of φωτιζω, to illumine, old verb from φως (Luke 11:36). If the sun and moon did shine, they would give no added light in the presence of the Shekinah Glory of God. See verse 11 for "the glory of God." Cf. 18:1; 21:3. "Their splendour is simply put to shame by the glory of God Himself" (Charles).

And the lamp thereof is the Lamb (κα ο λυχνος αυτης το αρνιον). Charles takes ο λυχνος as predicate, "and the Lamb is the lamp thereof." Bousset thinks that John means to compare Christ to the moon the lesser light (Genesis 1:16), but that contrast is not necessary. Swete sees Christ as the one lamp for all in contrast with the many λυχνια of the churches on earth (1:12,20). "No words could more clearly demonstrate the purely spiritual character of St. John's conception of the New Jerusalem" (Swete).