I was talking to my uncle today (who was baptized Catholic, but is now Buddhist), and he said he feels the Church is "arrogant" to say that she holds the fullness of the truth when so many other religions have "similar myths and legends" (virgins giving birth, a god impregnating a human, etc.). He said that he didn't understand why the Church couldn't just admit that she was one among many and that he didn't see any difference between Catholicism and the other religions.
I did talk about how, if all of creation was awaiting the coming of Jesus since the beginning, it makes perfect sense that there should be "echoes" of this in all of man's searching for god (thus the similar myths and things), but as far as how Catholicism is different from these...I just had no idea what to say or where to start. I know there are huge differences, but...any ideas on how I should have responded?
My Response (slightly edited--I always think of better ways to state things aftewards):
Prayer. That's the best response.
As for actual dialogue with your uncle, you could start with the Trinity (no other religion comes close to holding the idea of 3 persons in the one God), you could also try to distinguish the Incarnation (God retaining the full nature of God while taking on a full human nature, hypostatically united in one person) from the myths (half god/half man status, or fully god, but only seeming to be human, etc.).
You were right with the "echoes" idea. If Christianity is entirely true, that doesn't mean it has to be entirely unique. In fact, it likely means that Christianity is not entirely unique. It's extremely unlikely that no other religion would have ever come up with any correct doctrines, and that only the one true faith would have a single correct doctrine. Just because some other religions may have happened to have gotten certain aspects correct doesn't mean that Christianity copied them, it just means that they found pieces of the Truth, whereas the Catholic Church has received the fullness of the Truth. We, as Catholics, commend others (even atheists) in the places where they are correct. It is always good to urge others on toward goodness, truth, and beauty. God is the source of all of them. If someone truly strives for any of them, he will find himself growing closer to God and coming closer to realizing the fullness of Truth, which God has revealed. God established the Catholic Church to make sure that His revelation would be handed down accurately. Our claim of fullness of truth is not for the sake of self-righteousness; rather, it is for the sake of the Truth--something infinitely higher than elevating ourselves in our own (and other men's) esteem.