Mendelssohn only wrote the tune for that, the words are Charles Wesley. Wesley predated Mendelssohn by over a century, and he wanted that hymn sung to a much duller tune (apparently one of those used for his easter hymn Christ the Lord is Risen Today, but I am not sure which). Mendelssohn (and W.H. Cummings whose idea it was to adapt the tune) did Wesley a big favour there.