Preheat oven to 400ºF. Spoon a third of the marinara sauce on bottom of an oiled baking dish. Arrange ravioli to cover bottom then top each ravioli with a dollop of ricotta cheese. Layer one third of spinach and one third of mozzarella cheese. Repeat process until three layers are complete. Top lasagna with remaining cheese. Bake for 25 minutes, or until edges started bubbling. Broil on high for 3-4 minutes, or until top layer of cheese is golden brown. Allow to cool slightly before serving.