The hymn's theme is taken from references in the New Testament to the Christian being a soldier for Christ, for example II Timothy 2:3 ( KJV): 'Thou therefore endure hardness, as a good soldier of Jesus Christ.'
The piece became Sullivan's most popular hymn. The Salvation Army adopted the hymn as its favoured processional. Sullivan named the tune 'St Gertrude,' after the wife of his friend Ernest Clay Ker Seymer, at whose country home he composed the tune. The words were written by Sabine Baring-Gould in 1865, and the music was composed by Arthur Sullivan in 1871. ' Onward, Christian Soldiers' is a 19th-century English hymn.
Sabine Baring-Gould, 1869 Arthur Sullivan, c. Sheet music from The Hymnal Army and Navy
