The poem this is attributed to, by Roy Croft, is nearly identical to a German-language poem by Eric Fried titled "Ich liebe dich" ("I Love You"). Little is known about either poem, so it is difficult to say if Croft actually wrote this poem or if he translated Fried's poem and ended up with the credit. Often the quote is attributed to "Anonymous" or Elizabeth Browning, as well,
F. Scott Fitzgerald fell in love with a woman named Zelda, who he courted and pursued single-mindedly. At one point a female friend, named Isabelle, heard that the engagement had been called off, and wrote to him. In Fitzgerald's response was the above quote. The sentence following the quote, to Isabelle, reads "You're still a Catholic but Zelda's the only God I have left now." Later in 1920 Zelda and Fitzgerald married.
The character Jubal Harshaw says this line in Stranger. Interestingly, in the "Uncut" version of the book, the character does not speak this line. Many argue that Harshaw is the true main character of the book, as he consistently proffers the philosophies another character must deal with.
Emerson wrote this quote in an essay about friendship. The line that follows is, "Thine ever, or never." As a passionate, intellectual man, we see here in this quote a sharp tendency to state in no uncertain terms his intense feelings.