Consider it an overreaction to what Christians perceive to be a secular culture. . . Some people can take secularism in stride and see "good luck" as a phrase actually meaning "I wish you well" (a NICE comment!) Others see it as some sort of belief statement in random luck, which is theologically wrong.. . Anyway, anything secular that is said must be "pushed back" against, partly because it challenges their belief in divine providence (rather than mindless luck) and partly as a evangelistic tool and as part of "standing up for their faith." If they don't negate the comment, then they're being ashamed of their faith.. . It's usually not worth an argument in either direction. I would just try to accept their comments for what they are, and move on.. . Incidentally, someone else mentioned how the same Christians generally get offended by using the word "Xmas" instead of Christmas. I find this ironic, since the first two letters of Christ in Greek are chi ("X") and rho. So "X" was commonly used to designate Christ way back when the church began, not to remove him from the picture, but simply as a symbolic shorthand.. . Again, some Christians today view the "X" as antagonistic to the faith and as something to challenge openly.