I'd like to focus on two or three related objections that Graham Oppy raises to Christianity (or theism) in Four Views on Christianity and Philosophy (Zondervan, 2016).
1. Both here and in his monograph on The Best Argument Against God (Palgrave-Macmillian), Oppy makes simplicity a criterion for judging atheism to be preferable to Christianity. But there are basic problems with that appeal:
i) There's no doubt that simplicity can sometimes be a useful criterion to adjudicate between completing explanations. However, it's hard to justify simplicity as a general criterion. For instance, occasionalism is infinitely simpler than secondary causation. Just consider the gazillions of individual causes in the universe. Not just the sheer number, but different kinds of causes for different kinds of events, as well as elaborate causal chains, or intersecting causal chains. Secondary causality in the universe is fiendishly complex. By contrast, occasionalism posits a single agent for everything that happens. But obviously, Oppy rejects occasionalism, despite the fact that it's an immensely more parsimonious explanation.
Occam's razor isn't plausible purely in the abstract. Rather, that's something we can only judge on a case-by-case basis. Sometimes simplicity is a methodological virtue, but that's context-dependent.
ii) Simplicity isn't just one principle. There's the distinction between a simpler ontology and a simpler explanation. These can be in tension. Postulating more entities can simplify an explanation. For instance, physicists postulate subatomic particles to account for higher-level interactions.
iii) There's a metaphysical tradition that rejects the presumption of parsimony: the principle of plenitude. Leibniz is the best-known champion of that alternative. But it has a modern counterpart in theories of a multiverse. The principle is that anything that can happen will happen. It's a controversial claim, but hard to rule out a priori–or even a posteriori.
iv) Another basic problem with invoking Occam's razor is this: suppose we agree with Oppy that a world without God is simpler than a world with God. How does that contrast create any presumption that God doesn't exist?
At best, all it does is to note a consequence of a world with or without God. But how does noting that consequence make it more likely that one consequence is true while the other is false? It's just a logical relation between two things.
Suppose it's true that if God exists, the world will be more complex than if he doesn't exist. Assuming that's the case, how does that indicate that in fact we're living in a world where God does not exist? For if we were living in a world where God exists, then our world would be more complex. If God is real, then that consequences follows from his existence. Assuming that's the case, how does that observation provide any evidence that God isn't real?
2. Oppy says that alongside the miraculous birth of Jesus:
we can set reports of the miraculous births of Buddha, Krishna, Karna, Kabir, Zoroaster, Marduk, Horus, Romulus, Asclepius, Oedipus, Augustus Caesar, Qi, Lao-tse, and others.
…the many similarities between Christian miraculous births and miraculous births in other religions and traditions. Four Views on Christianity and Philosophy, 37-38.
There are several problems with his comparison:
i) It fails to distinguish between fictional characters, mythological gods, and historical figures. It stipulates parallels to the virgin birth rather than documenting parallels. But we'd need to see the details. And it fails to consider the genre of the accounts, or the date of the source in relation to the date of the individual. It's deceptive to call these "reports". That connotes an account which, at least in principle, had its basis in observation.
ii) More to the point, a basic way of assessing a claim is to ask yourself what would follow if the claim were true. If Jesus was virginally conceived, would that prevent other religions and traditions from having tales of gods, heroes, and founders whose conception was extraordinary? Since there'd be tales like this whether or not Jesus was virginally conceived, the existence of such tales doesn't tell against his virginal conception. The existence of such tales makes no difference one way or the other on whether Jesus was virginally conceived. In that respect, the situation would be just the same if he were virginally conceived. The virginal conception of Christ would be a fact regardless of what other stories might exist.
3. In the same book, Oppy automatically discounts testimonial evidence for miracles by appealing to the rapid development of urban legends (pp36-37,68-69). But that suffers from the same problem. Once again, ask yourself what would follow if the claim were true. If miracles do occur, then some miracles will be witnessed. And if miracles do occur, there will still have the phenomenon of urban legends. A world in which miracles occur won't eradicate urban legends. Urban legends would develop whether or not miracles actually happen. So how does the existence of urban legends discredit any and all reported miracles?
Testimonial evidence for miracles is just a subset of testimonial evidence in general. If urban legends create a presumption against reported miracles, do urban legends create a presumption against reported events generally? If not, why single out miracles as if the existence of urban legends only casts doubt on them?
4. Finally, his appeal to urban legends cuts both ways. You can have urban legends that attempt to explain away miracles. Take the cover story of the stolen body (Mt 28:11-15).