Yes, the context makes more sense if you see the movie or read the book.
A definition of materialism is the belief that all that exists is some form of matter or energy, which leaves no place for justice, mercy, duty, etc. Terry Pratchett seems to make the point that bringing justice, mercy, etc. involves an act of faith.
According to the link in the OP, “he rejects supernaturalism but considers himself culturally and morally Christian.” Is it possible to be a Christian and be an atheist at the same time?
If someone claims to believe in God but denies justice, mercy, and other values that Christ preached can they still be Christian?