At the end of the 18th century, composer Joseph Haydn visited England on several occasions. There he saw the results of that other musical superstart, Händel. He had risen to huge popularity in London by creating so called oratorios. Possibly Haydn saw one of these, and was inspired to created something similar.
The result was an oratorio telling the story of Genesis, mixed with fragments of the Psalms and Milton’s Paradise Lost. It was written in English and then translated into German. The German version still has a bit of an English rhythm to the language, but you basically come across this piece in two languages.
The difference between an oratorio and an opera is subtle. I can tell stories about the history of both forms of art, which are very different. On a more stylistic level I suppose one can say an oratorio is less stage play. There is less interaction between the characters along with a bigger role for the chorus. Opera is more musical theatre, where an oratorio is more like a concert with singers that serve different roles. Probably students of theatre can say much more about these differences.