“I write to reach eternity.”

On his inspiration, when he was still learning English and walking alone in the fields of Long Island, to take up the reading of No Cross, No Crown by William Penn, after having first set it aside upon realizing it was a religious book. In Memoirs of the Life and Gospel Labors of Stephen Grellet (1860), p. 20
Context: I was suddenly arrested by what seemed to be an awful voice proclaiming the words, "Eternity! Eternity! Eternity!" It reached my very soul — my whole man shook — it brought me like Saul to the ground. The great depravity and sinfulness of my heart were set before me, and the gulf of everlasting destruction to which I was verging. I was made to bitterly cry out, "If there is no God — doubtless there is a hell." I found myself in the midst of it.

Source: On Writing Well (Fifth Edition, orig. pub. 1976), Chapter 12, Writing About Yourself: The Memoir, p. 98.

Epigraph, Ch. 1 : Mount Shasta; this appears as "To Mount Shasta" in In Classic Shades, and Other Poems (1890), p. 126
Variant: I saw the lightning's gleaming rod
Reach forth and write upon the sky
The awful autograph of God.
This variant was cited as being in The Ship in the Desert in the 10th edition of Familiar Quotations (1919) by John Bartlett, but this appears to be an incorrect citation of a misquotation first found in The Japanese Letters of Lafcadio Hearn (1910), edited by Elizabeth Bislande, p. 161.
Shadows of Shasta (1881)
Context: Where storm-born shadows hide and hunt
I knew thee, in thy glorious youth,
And loved thy vast face, white as truth;
I stood where thunderbolts were wont
To smite thy Titan-fashioned front,
And heard dark mountains rock and roll;
I saw the lightning's gleaming rod
Reach forth and write on heaven's scroll
The awful autograph of God!

Drum-Taps. Bivouac on a Mountain-side
Bartlett's Familiar Quotations, 10th ed. (1919)

Hope, Faith, and Love (c. 1786); also known as "The Words of Strength", as translated in The Common School Journal Vol. IX (1847) edited by Horace Mann, p. 386
Context: There are three lessons I would write, —
Three words — as with a burning pen,
In tracings of eternal light
Upon the hearts of men. Have Hope. Though clouds environ now,
And gladness hides her face in scorn,
Put thou the shadow from thy brow, —
No night but hath its morn. Have Faith. Where'er thy bark is driven, —
The calm's disport, the tempest's mirth, —
Know this: God rules the hosts of heaven,
The habitants of earth. Have Love. Not love alone for one,
But men, as man, thy brothers call;
And scatter, like the circling sun,
Thy charities on all. Thus grave these lessons on thy soul, —
Hope, Faith, and Love, — and thou shalt find
Strength when life's surges rudest roll,
Light when thou else wert blind.

Burning the Days (1997 memoir)

Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 213.