This is a controversial statement, because English literature presupposes both the Bible and classical literature. However, if I had to pick one, I would say that the Biblical background is more important. Let me give you a few reasons why.
First, from the point of view of worldviews, English literature was started by people whose common sense view of the world was filled with the Bible, whether they were religious or not. In other words, Europe was shaped by Christianity. So even if an author revolted from Christianity, it was based on a biblical world view. We can say that it was a reaction against Christianity.
Second, as important as the classical world was for Europe, Christianity in time took over and more importantly, classical literature was mediated through the lens of Christianity. For this reason also, Christianity is essential as a background to English literature.
In light of these points, we would do to remember the importance of Christianity in reading English literature.