Answer: a lot. Like, a whole lot. The following isn’t at all definitive, but it should give you a sense of the historical milieu from which the song came.
I think all of us have a vague idea that “The Battle Hymn of the Republic” has something to do with the Civil War, and you may or may not be aware that its tune is the same as “John Brown’s Body.” a popular song among Union soldiers that they had adapted from an old Methodist revival song, “Say, Brothers, Will You Meet Us.” Wikipedia says this about the origin of the words, taken from an account by a soldier::
We had a jovial Scotchman in the battalion, named John Brown. … and as he happened to bear the identical name of the old hero of Harper’s Ferry, he became at once the butt of his comrades. If he made his appearance a few minutes late among the working squad, or was a little tardy in falling into the company line, he was sure to be greeted with such expressions as “Come, old fellow, you ought to be at it if you are going to help us free the slaves,” or, “This can’t be John Brown—why, John Brown is dead.” And then some wag would add, in a solemn, drawling tone, as if it were his purpose to give particular emphasis to the fact that John Brown was really, actually dead: “Yes, yes, poor old John Brown is dead; his body lies mouldering in the grave.”
Julia Ward Howe, a poet and abolitionist, visited a review of the troops along the Potomac in 1861 after the start of the war, and she heard the soldiers singing the JBB song, joining in with them as her carriage moved back to Washington. (She apparently had a very nice singing voice, and the soldiers applauded her.) A clergyman encouraged her to write some better words to the tune, and this is what she says happened the next morning:
I went to bed that night as usual, and slept, according to my wont, quite soundly. I awoke in the gray of the morning twilight; and as I lay waiting for the dawn, the long lines of the desired poem began to twine themselves in my mind. Having thought out all the stanzas, I said to myself, “I must get up and write these verses down, lest I fall asleep again and forget them.” So, with a sudden effort, I sprang out of bed, and found in the dimness an old stump of a pen which I remembered to have used the day before. I scrawled the verses almost without looking at the paper.
It’s particularly appropriate that a song about John Brown should have been the inspiration for Howe’s own lyrics, because her husband, Samuel Gridley Howe, had helped fund John Brown’s activities, including Brown’s raid on Harper’s Ferry that resulted in his hanging. The group of Brown’s supporters was called the Secret Six. (I myself have eaten a couple of very good meals at the “Secret Six Tavern” in Harper’s Ferry; be sure to visit that charming town if you’re ever in the area.) Howe’s lyrics were first published in The Atlantic Monthly in 1862 and quickly became an unofficial anthem for the Union side.
What of the words themselves? Two general points can be made. First, they are steeped in biblical imagery, and second, they depict the Union army as carrying out the will of God. After the Civil War was over the song morphed into something more generally patriotic and stirring. Let me point out a couple of especially striking lines drawn from Scripture:
“Mine eyes have been the glory of the coming of the Lord/He is trampling out the vintage where the grapes of wrath are stored.”
The source of these words can clearly be traced to several passages in the Book of Revelation in which “the winepress of God’s fury” is described as judgment comes on the earth. Howe came up with the memorable phrase “the grapes of wrath,” thus giving John Steinbeck the title for his novel.
She specifically references the Union soldier camps with the lines:
“I have seen Him in the watchfires of a hundred circling camps/They have builded Him an altar in the evening dews and damps/I can read His righteous sentence by the dim and flaring lamps.”
In other words, God is present with the Union cause, and the troops are carrying out His will, His “righteous sentence,” as they fight the slaveholding South. Julia is taking the idea of the final judgment at the end of the world and applying it to a specific event in American history, an action that is understandable given her abolitionist views but questionable in terms of theology. After all, Lincoln himself famously said, “Sir, my concern is not whether God is on our side. My great concern is to be on God’s side.”
I have always found the final verse with its words “In the beauty of the lilies Christ was born across the sea” to be rather puzzling. As someone asked in an online forum, “I thought the story was that he was born in a manger, not among lilies.” (You can read the whole thread here; it’s quite interesting.) While we can never know for sure what prompted Julia to write those specific words—since, as far as I know, she never gave a line-by-line analysis of her lyrics—we can only speculate. Howe may very well be referencing several “lily” images:
1) The Old Testament/Jewish Bible book Song of Solomon 2:1: “I am the Rose of Sharon and the lily of the valleys,” which some later Bible scholars saw as a reference to the Messiah.
2) The statement from Jesus’ Sermon on the Mount in the Gospel of Matthew: “Consider the lilies of the field, how they grow; they toil not, neither do they spin: And yet I say unto you, That even Solomon in all his glory was not arrayed like one of these.” (6:28b-29 KJV)
3) The Easter lily, associated with Christ’s resurrection.
Some versions render the next-to-last line of this verse, “As He died to make men holy let us live to make men free,” but others have it as “let us die to make men free,” which fits better with the war imagery of the original.
The chorus of “Glory, glory, Hallelujah” almost certainly comes from the Book of Revelation chapter 19, the same passage that inspired Handel’s “Hallelujah Chorus”:
And I heard as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying, Alleluia: for the Lord God omnipotent reigneth. (19:6 KJV)
From there is was an easy step for Howe to change the original “But his soul is marching on” to “His truth is marching on.” (Although some versions do say “truth, as in the video below.)
Of course there are about a million performances/arrangements/versions on YouTube. I’ve chosen one by the Mormon Tabernacle Choir as being appropriately majestic:
And here’s a performance of “John Brown’s Body” with some great historical illustrations. Note that the singer says that Brown’s body is “a-moldin'” in the grave. She uses the “his truth is marching on” wording:
And just for fun–with more cool historical illustrations–a rendering of “Say Brothers Will You Meet Us?”–
And here are the official lyrics:
He is trampling out the vintage where the grapes of wrath are stored;
He hath loosed the fateful lightning of His terrible swift sword;His truth is marching on.
Glory! Glory! Hallelujah!
Glory! Glory! Hallelujah!
His truth is marching on.
They have builded Him an altar in the evening dews and damps;
I can read His righteous sentence by the dim and flaring lamps,
His day is marching on.
“As ye deal with my condemners, so with you My grace shall deal!
Let the Hero, born of woman, crush the serpent with his heel, ”
Since God is marching on.
He is sifting out the hearts of men before His judgment seat;
Oh, be swift, my soul, to answer Him; be jubilant, my feet!
Our God is marching on.
With a glory in His bosom that transfigures you and me;
As He died to make men holy, let us die to make men free!
While God is marching on.