And the gates of it shall not be shut at all by day: for there shall be no night there. -- Revelation 21:25 (KJV)
Ancient cities shut their gates at sundown. That was not ceremony. It was the difference between a night you sleep through and one you don't — raiders, animals, whoever had been waiting outside for the light to go.
So when John reports that the gates of this city are never shut, he gives the reason in the same breath, and it has nothing to do with thicker walls. There shall be no night there.
Where the Light Comes From
The city had no need of the sun, neither of the moon, to shine in it. Both still exist in the sentence; both are described as unnecessary. The glory of God did lighten it.
Every light you have read by was borrowed. The sun burns through its fuel, the lamp draws current, the screen is lit from behind. Here the light has no source outside itself, which means it cannot fail, dim, set, or go out in a storm. And when the sun goes, so does the cycle it runs — evening, night, morning — that has organized every day you have ever lived.
What Does Not Get In
An open gate makes people nervous for a reason, so John says what the arrangement rests on.
There shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie.
The gates can stay open because nothing that needs the dark is coming through them. Then the chapter finishes its own sentence: but they which are written in the Lamb's book of life. That is the entry requirement in full — a name on a list, not a record you assemble.
A city in permanent daylight has nowhere to put the parts of a life that only work unobserved. If that reads as relief, good. If it reads as exposure, that is something to find out now, while there is still an evening to think it over.
