"Blessed Assurance" was written by Fanny Crosby (married full name, Frances J. Van Alstyne) - according to Wikipedia -

Crosby was visiting her friend Phoebe Knapp as the Knapp home was having a large pipe organ installed. The organ was incomplete so, using the piano, Mrs. Knapp played a new melody she had just composed. "What do you think the tune says?" asked Knapp.

"Blessed assurance, Jesus is mine", answered Fanny Crosby.

They don't make 'em like that any more!
Chris

ps - Google Books has some great Welsh hymnals - See Odlau'r Efengyl -
18885 - great Welsh words by Watcyn Wyn to the great revival hymns.