Blessed Assurance is a well-known Christian hymn. The lyrics were written in 1873 by blind hymn writer Fanny Crosby to the music written in 1873 by Phoebe Knapp.

Crosby was visiting her friend Phoebe Knapp as the Knapp home was having a large pipe organ installed. The organ was incomplete, so Mrs. Knapp, using the piano, played a new melody she had just composed. When Knapp asked Crosby, "What do you think the tune says?", Crosby replied, "Blessed assurance; Jesus is mine."
The hymn appeared in the July 1873 issue of Palmer's Guide to Holiness and Revival Miscellany, a magazine printed by Dr. and Mrs. W. C. Palmer of 14 Bible House, New York City. 
The popular song reflects Crosby's walk of faith, as expressed by the apostle Paul in Philippians 1:21, "For to me, to live is Christ and to die is gain"

Lyrics:
Stanza 1
Blessed assurance, Jesus is mine!
O what a foretaste of glory divine!
Heir of salvation, purchase of God, 
Born of His Spirit, washed in His blood.

Refrain:
This is my story, this is my song, 
praising my Savior all the day long; 
this is my story, this is my song, 
praising my Savior all the day long. 

Stanza 2
Perfect submission, perfect delight! 
Visions of rapture now burst on my sight; 
Angels descending bring from above 
Echoes of mercy, whispers of love.
(Refrain)

Stanza 3
Perfect submission, all is at rest! 
I in my Savior am happy and blessed, 
Watching and waiting, looking above, 
Filled with His goodness, lost in His love. 
(Refrain)