While doing genealogy work some years ago, I came across an epitaph in a New England cemetery where several of my ancestors are buried. I remember I was somewhat shocked to read:
As you are now, so once was I
As I am now so you will be
Prepare for Death to follow me. 
Later I learned that this was a common sentiment found on early generations of New England gravestones. This message appealed to the New England Puritans because it emphasized the seriousness of life and the resultant need for self-examination and preparation for eternity. Another version of this grave inscription from colonial days is: “Stranger, stop and cast an eye/ As you are now, so once was I/ As I am now, so you will be/ Remember Death and follow me.” Or, on other gravestones, “Death is a debt/ To Nature due/ That I have paid/ And so must you.” These maxims point out the inevitability of death for all who dwell in this sphere.
As I traced the provenance of this epigraph I found similar writings in churchyards throughout Great Britian. Here’s one from an English tombstone from the East Sutton church above the Kentish Weald in Kent:
“Remember me as you pass by,
As you are now, so once was I.
As I am now, so you will be.
Remember Death will follow thee.”
And among one of Ireland’s most popular epitaphs:
Dear Friend As You Pass By
As You Are Now So Once Was I.
As I Am Now So You Must Be
Prepare The Way To Follow Me.
This thought is said to come from the Irish proverb: “Hodie mihi, cras tibi” which is translated “My turn today, yours tomorrow.”
And even further back in time, we discover the maxim “Eram quod es; eris quod sum,” coined by Horace. It may be translated, “I was what you are; you will be what I am.”
That the majority of Latter-day Saints were unacquainted with the prevalent use of these words on gravestones became amusingly evident last year when a wikipedia article was nominated on the Mormon site “Sustain’d.” This article described The Holy Trinity, a famous fresco by the Early Italian Renaissance painter Masaccio. It is located in the church of Santa Maria Novella, in Florence, painted between 1425 and 1428. Below the fresco lies a sarcophagus upon which is inscribed the words: I WAS WHAT YOU ARE AND WHAT I AM YOU SHALL BE. (translated from the Latin) Mormons interpreted the inscription to be a 600-year-old confirmation of Mormon doctrine.
By now you have recognized that I am referring to the couplet coined by Lorenzo Snow,
“As man now is, God once was:
As God now is, man may be.”
There is something about certain literary phrases which lend themselves to the exposition of doctrine. This seems to be one of them. Lorenzo was not the only follower of religion to use this convention to express ecclesiastical truths. A maxim which is repeated in the Catholic church goes like this:
What we are, you once were.
What we believe, you once believed.
How we worship, you once worshipped.
If you were right then, we are right now.
If we are wrong now, you were wrong then.
These words are used by Catholics to reinforce their support for tradition. Most recently it has been cited to defend the latest motu proprio by Pope Benedict XVI relating to the traditional liturgy. (i.e. that the administering of most of the sacraments should be in the form prior to the liturgical reforms after the Second Vatican Council.) It can also mean that either the Catholics are right in their claim to authority, or that Catholics and Protestants are all wrong together, as noted by Orson F. Whitney. 
The doctrine behind Lorenzo Snow’s couplet was said to have originated with Joseph Smith Sr. At a blessing meeting, the Patriarch Father Smith told the young Lorenzo that he would soon be convinced of the truth of the latter-day work, and be baptized–”You will become as great as you can possibly wish — EVEN AS GREAT AS GOD, and you cannot wish to be greater.”  These words worked upon Lorenzo’s mind until he received the famous couplet as a revelation. In January, 1843, Lorenzo Snow related to the Prophet Joseph Smith his experience in a confidential interview in Nauvoo. The Prophet’s reply was: “Brother Snow, that is a true gospel doctrine, and it is a revelation from God to you.”  Joseph himself publicly taught the doctrine the following year, 1844, during a funeral sermon of Elder King Follett: “God was once as we are now, and is an exalted man, and sits enthroned in yonder heavens…It is the first principle of the gospel to know for certainty the character of God and to know that we may converse with him as one man with another, and that he was once a man like us; yea, that God himself, the father of us all, dwelt on an earth, the same as Jesus Christ did. Here then, is eternal life — to know the only wise and true God; and you have got to learn to be Gods yourselves, and to be kings and priests to God, the same as all Gods have done before you.” 
We have often seen in the Church how revelation can come through the medium of things that lie within our daily experience. Perhaps Lorenzo Snow was acquainted, at least in a subliminal sense, with the “eram quod es” sentiment. Certainly the epitaph had found its way to Ohio and Illinois cemeteries as well as those of New England.  His family environment also gave him a strong background in poetry (his sister being Eliza R. Snow!) Thus he was able to formulate revelatory thoughts which came to him into a poetic form. The catchy couplet in turn captured Joseph Smith’s attention and was transferred into doctrine.
The couplet and the doctrine of eternal progression has been taught in many venues in the Church throughout the years, including in official Church publications,  the Ensign,  and in General Conference.  Inclusion of the relevant portion of the King Follett Discourse in our latest “Prophets” manual, while not canonizing the teaching, certainly “correlates” it. Which makes it as close to official as we get in the Church today.
After Lorenzo’s revelation and the Prophet’s approbation in his formal teachings, and after the doctrine of eternal progression had been taught for many years, President Snow (1892) expanded the couplet into a poem responding to John’s writings in the book of Philippians. This poem is extremely long, and, written in couplet form becomes rather monotonous. It is often shortened to the portion you see here.
By Lorenzo Snow
“Let this mind be in you, which was also in Christ Jesus, who, being in the form of God, thought it not robbery to be equal with God.” - Philippians 2:5,6
Hast thou not been unwisely bold,
Man’s destiny to thus unfold?
To raise, promote such high desire,
Such vast ambition thus inspire?
Still, ’tis no phantom that we trace
Man’s ultimatum in life’s race;
This royal path has long been trod
By righteous men, each now a God:
As Abra’m, Isaac, Jacob too,
First babes, then men- to gods they grew.
As man now is, our God once was;
As now God is, so man may be,-
Which doth unfold man’s destiny.
. . . . . . . . .
The boy, like to his father grown,
Has but attained unto his own;
To grow to sire from state of son,
Is not ‘gainst Nature’s course to run.
A son of God, like God to be,
Would not be robbing Deity;
And he who has this hope within,
Will purify himself from sin.
You’re right, St. John, supremely right:
Whoe’er essays to climb this height,
Will cleanse himself of sin entire-
Or else ’twere needless to aspire. 
 from the gravestone of Mrs. Betty Johnson, d. 4 Dec 1799. At Burial Hill, Plymouth MA, a cemetery established ca 1717.
 Orson F. Whitney, *Saturday Night Thoughts, Part 3,* (Salt Lake City: Deseret News Press, 1921), 63-64. Orson F. Whitney quotes a Catholic theologian as saying: “The issue is between Catholicism and Mormonism. If we are right, you are wrong; if you are right, we are wrong; and that’s all there is to it. The Protestants haven’t a leg to stand on. If we are wrong, they are wrong with us, for they were a part of us and went out from us; while if we are right, they are apostates whom we cut off long ago.” see also Footnotes on the Strength of the Mormon Position at BCC.
 Eliza R. Snow, Biography and Family Record of Lorenzo Snow, Salt Lake City: Deseret News Co., 1884, pp. 9–10.
 LeRoi C. Snow, Improvement Era, June 1919, p. 656.
 Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith, Salt Lake City: Deseret Book, 1938, pp. 345–46.
 see, for example, Zion’s Lutheran Cemetery, Obetz, Ohio.
 Encyclopedia of Mormonism 4:1474. “This process known as eternal progression is succinctly expressed in the LDS aphorism, ‘As man is, God once was. As God is, man may become.”
 Gerald N. Lund, “I Have a Question,” Ensign, Feb. 1982, 39–40. “This particular doctrine has been taught not only by Lorenzo Snow, fifth President of the Church, but also by others of the Brethren before and since that time…Numerous sources could be cited, but one should suffice to show that this doctrine is accepted and taught by the Brethren. In an address in 1971, President Joseph Fielding Smith, then serving as President of the Quorum of the Twelve Apostles, said: ‘I think I can pay no greater tribute to [President Lorenzo Snow and Elder Erastus Snow] than to preach again that glorious doctrine which they taught and which was one of the favorite themes, particularly of President Lorenzo Snow. …’”
 President Gordon B. Hinckley, General Conference, October 1994. “On the other hand, the whole design of the gospel is to lead us onward and upward to greater achievement, even, eventually, to godhood. This great possibility was enunciated by the Prophet Joseph Smith in the King Follet sermon; and emphasized by President Lorenzo Snow. It is this grand and incomparable concept: As God now is, man may become!”
 Improvement Era 22:660-661, June 1919. Originally written January 11, 1892.