How should I prepare for a theology interview at Oxbridge?

The best practical advice I could give would be to ensure that you know your personal statement inside-out. At Cambridge they tend to give 2 interviews where one of the interviews will focus primarily upon what you put in your personal statement and any natural questions which may flow from this. In this interview, the first interview will effectively act as a stimulus or spring-board from which they will ask you questions in order for them to try and unpick your brain, and see how you think!
In the second interview, it tends to be a more abstract and 'unpredictable' interview from what I and others I asked at Cambridge experienced. In these interviews, a stimulus such as an unseen source/text will be presented to you 15 mins before interview in which they will later ask you to talk about. This exercise is very similar to a kind of "unseen poetry" question that you may have done in English. Here they are trying to see how you react to unfamiliar and challenging material and try and push you to think deeper about questions. This is supposed to mirror the kind of teaching that you will have in supervisions at Cambridge where the bulk of the teaching is done via supervisions where you meet with a supervisors in pairs to discuss a particular book/text that you will have been asked to write an essay on. Here they are testing how well you are able to adapt to this kind of method of teaching, and whether you would be a suitable student who would be receptive to this. After all, they are looking for someone who they are going to want to teach for an hour a week, for 8 weeks every term. This way they are able to test whether you are a good fit for this.
Generally, the interview is part of a holistic process where grades and the personal statement application are all taken into consideration. I have had many friends recall stories about how terrible they thought their interviews went, despite being successfully accepted. With theology interviews and humanities in general, they want to see passion for your subject. They are not looking to test how much you know or have read, but how much you love and are willing to read for love of the subject you want to study. This is why the questions don't really look for a "right answer' per se, the best kind of answers are those which show real thought to them, where unique and interesting interpretations to often very vague and obscure questions, are welcomed!