Literature is often full of symbolism: words, symbols, or other
things that evoke images or allude to outside influences without
the authour having to specifically mention the meaning of the
symbol. For example, heroes in certain stories often share
similar traits, most likely due to influences on their
characters from previous tales. The Bible, being a well-known
piece of literature (at the very least, it is a book) is often
the source of many influences that modern-day writers draw on,
particularly with heroes in certain kinds of tales being
required to sacrifice something - sometimes their life - to save
the world. It's obvious that Hoshino Katsura is drawing on at
least one of those tales - Noah's Ark - for D.Gray-Man, but for
a story set in an Order under the control of the Vatican, there
is, as one of the posters here (=P) has commented, remarkably
little mention of Catholic beliefs and figures in the story.
However, we know Hoshino does her research, and there are a few
things that stood out right away