Fanny Crosby (1820-1915) is heralded as one of the world's most prolific and talented hymn composers. Blind from shortly after birth, Fanny nonetheless wrote 8,000 hymns.

One day Fanny overheard her friend and fellow musician Phoebe Knapp playing a new composition on the organ. Phoebe told Fanny the tune was called Assurance. Fanny replied: 'Blessed Assurance! Jesus is mine!' And so began the collaboration that was to become one of Christendom's most beloved hymns. Blessed Assurance was published in 1873.

(http://www.sharefaith.com/guide/Christian-Music/hymns-the-songs-and-the-stories/blessed-assurance-the-song-and-the-story.html)

This text and 'To God Be the Glory" (473) are probably the best known texts by Fanny J. Crosby (PHH 473). She said the following about her writing of the text:

Sometimes a tune is furnished me for which to write the words. The hymn titled "Blessed Assurance" was made in this manner. My dear friend Phoebe Palmer Knapp (Mrs. Joseph), so well-known as a writer and singer of most excellent music and as an aid and inspiration to all who knew her, had composed the tune; and it seemed to me one of the sweetest I had heard for a long time. She asked me what it said. I replied, "Blessed assurance." I felt while bringing the words and tones together that the air and the hymn were intended for each other
-from Fanny Crosby's Memories

Crosby's text and Knapp's tune were published in John R. Sweney's Gems of Praise in 1873. The hymn was also published in both the American and British editions of the Ira D. Sankey (PHH 73) hymnals and was more recently featured in Billy Graham Crusades. It is an immensely popular hymn in English-speaking Christendom. The Psalter Hymnal includes the original stanzas 1 and 3, but stanza 2 by Marie J. Post (PHH 5) is substituted for Crosby's "rapture" stanza.

Blessed Assurance" is a typical gospel hymn of the late nineteenth century. It is simple, truly evangelical in spirit, and has an emotional appeal that comes from its rousing tune and from the personal experience described in the text. It is a fine testimonial hymn of praise to Christ for his work of redemption (st. 1), for the Spirit's work of sanctification (st. 2), and for the joy of serving Jesus (st. 3).

(http://www.hymnary.org/text/blessed_assurance_jesus_is_mine)

http://www.hymnary.org/person/Crosby_Fanny