Augustine before his conversion
Christians believe that the soul is eternal and always exists, either in heaven or in hell.
Now, the soul is raceless and according to them the only important thing about a man; the body is dross, merely a shell. Why should someone who believes he is raceless and never going to die, and who is focused intently on things not of this world, but an anticipated next, worry himself about trivial matters such as the survival of Western civilization or of this or that biological race?
Clearly, to do so would be a heresy.