You shouldn't forget that the bible is not the only source for alusions in European literature. In my opinion knowledge about Greek mythology is far more important to interpret German literature than knowledge about the bible. But expect the few students that learn ancient languages hardly anyone knows anything about it. Despite this you will not see a big difference between the students who study Latin and the ones who don't.

Despite all of this I do not think that religious knowledge should be required to understand literature at high school level. Of course it offers some deeper insight if you know that the whole storyline of Faust is a variation of Job, that the portrayal of Mephistopheles contradicts the church's image of the devil or that Goethe laid his own pantheistic and often anti religious views into Faust's mouth, but in my experience the teacher will provide the corresponding secondary sources (e.g. an extract from the bible, mythological background, etc.).