We cannot do Vatican II theology without addressing that challenge. When you get stuck try one of the following. Re-read the first paragraph. Re-read the first full paragraph on page 49. Read the first full paragraph on page 53 before read the whole essay.
Ontology refers to existence and to being.
In your philosophy class you would have considered several notions of ontology. Recall the difference between Plato and Aristotle for example. Write the questions that emerge from this reading or the Rahner material in general. Come prepared to read them in class and to hand them in to me. It is different from the Rahner material in that it is more straightforward and describes history more than method in theology.