When someone has a strong opinion on a topic or believes with conviction a certain way...not only regarding religion, they may came across, unintentionally as being smug or unreasonable, perhaps even condescending when you present your view to them. This is easier to see in person.
On a message board though there are always those who like to project themselves, their image, as being more intelligent, more wordly than they actually are. They can research subject matter, formulate what they want to say, then post it. Having an actual conversation with them would be much different. They tend to talk down to people, trying to make them feel stupid when they post.