This hymn tune is called "St. Gertrude" in hymn books and was composed by Sir Arthur Sullivan of "Gilbert and Sullivan" fame (to words written by Sabine Baring-Gould). Compare bars 1 and 5 (and to a lesser extent bars 2 and 6) and notice that the parts for the two hands have changed places! The Salvation Army (who do excellent and much needed work with homeless people) seem to have adopted this hymn, probably because of the symbolism in a Christian "army" of "soldiers" spreading the word.
The audio controls below allow you to play the mp3 version of Onward Christian Soldiers by Arthur Sullivan for piano or you can download the MP3 file. You can also download the midi version of Onward Christian Soldiers by Arthur Sullivan for piano or alternatively edit/play the midi file.
The image below is the first page of the sheet music, and here you can download the full sheet music for Onward Christian Soldiers by Arthur Sullivan for piano in PDF format.