Choral music was founded, nurtured, and developed almost exclusively within the Christian church for hundreds of years. To participate in choral singing is to participate in a historically Christian art form. If you choose to sing in a choir and do it with an accurate representation of the tradition, you will sing a lot of sacred Christian music.
There are countless instances of secular and non-Christian choral music (including many examples of Jewish choral music), but the vast majority of choral music throughout history is of a sacred Christian nature.
In addition, many of the most important works in the canon are of Christian derivation. If it were not for the Christian Church, we probably would not have choral singing as we know it.
from “Choral Journal,” vol. 56