The University of British Columbia (link) has a good discussion of this painting which was inspired by the poem "Sir Galahad," by Alfred, Lord Tennyson (1834). A longer excerpt is below.
When down the stormy crescent goes, A light before me swims,
Between dark stems the forest glows,
I hear a noise of hymns:
Then by some secret shrine I ride;
I hear a voice but none are there;
The stalls are void, the doors are wide,
The tapers burning fair.
Fair gleams the snowy altar-cloth,
The silver vessels sparkle clean,
The shrill bell rings, the censer swings,
And solemn chaunts resound between.
I find a magic bark;
I leap on board: no helmsman steers:
I float till all is dark.
A gentle sound, an awful light!
Three angels bear the holy Grail:
With folded feet, in stoles of white,
On sleeping wings they sail.
Ah, blessed vision! blood of God!
My spirit beats her mortal bars,
As down dark tides the glory slides,
And star-like mingles with the stars.
When on my goodly charger borne
Thro' dreaming towns I go,
The cock crows ere the Christmas morn,
The streets are dumb with snow.
The tempest crackles on the leads,
And, ringing, springs from brand and mail;
But o'er the dark a glory spreads,
And gilds the driving hail.
I leave the plain, I climb the height;
No branchy thicket shelter yields;
But blessed forms in whistling storms
Fly o'er waste fens and windy fields.
