A brief history: Pentecost Monday was a special day for schools in Yorkshire (England). 

There were a variety of activities. The pupils of Sabine Baring-Gould would meet with the children of a nearby village. It seemed like a good idea that during the walk would be sung. But she could not find a suitable song and decided to write one by herself. "Onward Christian Soldiers" was the result. 

It soon became very popular, though she herself was not entirely satisfied with the rhyme scheme. The melody used (St. Gertrude) was by the famous English composer Sir Arthur S. Sullivan. 

Back to now: Gawan Roberts, has given the hymn a proper update. He gave the still popular song a solid rock beat and added daring harmonies. So it's popular, for current generations, again for years. Onward!