Christianity adds lots of new religious ideas and practices to ancient paganism. But does it add anything philosophical? Christian thinkers from the very start do lots of very interesting and excellent conceptual work defending and justifying the particular creeds and activities of the Christian faith. But that work is theological rather than philosophical. If your reasoning relies on premises that are justified only by religious revelation or religious authority, it’s theology, not philosophy. I love theology, but it’s not philosophy. So, philosophically speaking, what does Christianity add?
The familiar arguments for the existence of God were all present in ancient pagan writers. Socrates developed the first design argument (it’s in Xenophon’s Memorabilia). Plato developed the first cosmological argument in the Timaeus. The Stoics further develop the design arguments and the arguments from degrees of perfection. Cicero already has a well-developed concept of God as the maximally perfect being. When these arguments were presented, even in the context of pagan monotheism, they weren’t arguments for the Christian God (or for any other Abrahamic God). They were arguments for Gods that belonged to pagan systems of thought. The Platonists had pretty sophisticated arguments for the existence and immortality of the soul. Discussions of free will and determinism were present in the Stoics (especially Epictetus). In his discussions of reincarnation, Plotinus deals with justice and life after death.
Christian thinkers seem to make two contributions to the philosophy of religion. The first is the Anselmian ontological argument. This argument really is new. But you can find reasoning that is very close to it in Book Two of Cicero’s On the Nature of the Gods. The second thing is the idea that God is a necessary being. So probably modal versions of the cosmological argument (like Aquinas’s third way or Leibniz’s sufficient reason argument) are new. But even here there are intimations of divine necessity in Plotinian arguments for the logical priority of the One. The question now arises: granted that these arguments and ideas were first developed by Christian philosophers, are they in any way distinctively Christian? Or could pagan philosophers have easily incorporated them? And the answer is clearly that the pagans could have easily incorporated them. The Stoics would have welcomed Anselm’s argument. The Neoplatonists could have easily added a plurality of worlds with the One as a necessary ground.
It’s easy to talk about Athens versus Jerusalm or about the God of the Philosophers versus the God of Abraham, Isaac, and Jacob. It’s easy to say that the Medieval schoolmen baptized Aristotle (and Plato too). And it’s hard to draw any clear line between philosophy and theology. Obviously Christianity contributed an extremely rich system of religious and theological ideas to the pagan philosophical world. But the question remains: has Christianity contributed anything new to the philosophy of religion? Or is it just elaborating and extending old pagan ideas? There’s nothing wrong with elaborating and extending old ideas, philosophers do it all the time. But give credit where credit is due. And at least talk about the problem raised by the fact that the main arguments for God were not arguments for the Christian God. The ancient pagan philosophers weren’t trying to be Christians.
It would be absurd to start the history of philosophy with Augustine, as if the entire pagan world never existed. But I have yet to see a single philosophy of religion textbook which recognizes that almost every argument for the Christian God started out as an argument for a non-Christian God. Pagan philosophy, including pagan philosophy of religion, was active for almost one thousand years in the West. One thousand years! I have yet to see a single philosophy of religion textbook that even discusses pagan philosophy of religion as such (I'm sure I haven't seen every philosophy of religion textbook, but I've seen lots of them). So why doesn’t it appear in the philosophy of religion textbooks?