Religion is not as concrete as Nature. The elements of Nature are unpredictable yet stable. Palestine is the grave of Jesus. We live in nature and it is unpredictable. Humans can depend on nature.
Nature is a constant.
Nature is unpredictable and free. Nature is inescapable. This poem is about religion versus nature. It describes reasons and circumstances that would lead to nature having the same validity as Christianity. Sunday Morning sees nature as having the elements that religions believe that God has.