Yes, I see where you are coming from. I misspoke when I said it could never not be offensive. However, even if you use this definition of ignorant, in this context it suggests that people who are Christians choose not to, or are incapable of, learning whatever the "right" thing is. This is totally fine if you are a person who believes Christians are misled or misinformed, but you have to see how they can take that as a personal offense--and religion is always a raw nerve. Something completely innocuous almost always becomes offensive when it comes to religion, which is why I tend to steer clear. It's damn near impossible not to offend.