“The gates of hell are open night and day;
Smooth the descent, and easy is the way:
But to return, and view the cheerful skies,
In this the task and mighty labor lies.”

The quote "The gates of hell are open night and day; Smooth the descent, and easy is the way: But …" is famous quote by Virgil (-70–-19 BC), Ancient Roman poet.

Facilis descensus Averno:
Noctes atque dies patet atri ianua Ditis;
Sed revocare gradum superasque evadere ad auras,
Hoc opus, hic labor est.
Variant translation:
: It is easy to go down into Hell;
Night and day, the gates of dark Death stand wide;
But to climb back again, to retrace one's steps to the upper air—
There's the rub, the task.
Compare:
Long is the way
And hard, that out of Hell leads up to Light.
John Milton, Paradise Lost, Book II, line 432
Source: Aeneid (29–19 BC), Book VI, Lines 126–129 (as translated by John Dryden)