Frederick M. Lehman, author and composer, wrote a pamphlet, in 1948, entitled History of the Song, The Love of God. It tells about the origin of this beloved hymn—

While at camp-meeting in a mid-western state, some fifty years ago in our early ministry, an evangelist climaxed his message by quoting the last stanza of this song. The profound depths of the line moved us to preserve the words for future generations.

Not until we had come to California did this urge find fulfillment, and that at a time when circumstances forced us to hard manual labor.

One day, during short intervals of inattention to our work, we picked up a scrap of paper and, seated upon an empty lemon box pushed against the wall, with a stub pencil, added the (first) two stanzas and chorus of the song.

Since the lines (3rd stanza from the Jewish poem) had been found penciled on the wall of a patient’s room in an insane asylum, the general opinion was that this inmate had written the epic in moments of sanity.

Actually, the key-stanza (third verse) under question as to its authorship was written nearly one thousand years ago by a Jewish songwriter, and put on the score page by F.M. Lehman, a Gentile songwriter, in 1917.

Source: http://www.tanbible.com/tol_sng/sng_theloveofgod.htm