Having an offensive blog isn't rude. Annoying? Possibly. Childish? This one sounds like it. But it's not rude.
I totally disagree with the statement I bolded. I'm not saying the guy doesn't have a right to hold the opinions, or blog about them, but that doesn't make his opinions and especially the way he words them polite and if they aren't polite, they are rude.
A person is free to have an opinion I disagree with and that isn't rude. He can state his reasons for his opinion and for disagreeing with my opinion and that isn't rude. When that person starts being insulting and calling me names for holding the opinion I hold, that is rude. He may have the right to do it, but that doesn't make it polite.
If he's personally insulting you, then that's rude. But as long as there is no direct and personal attack, I refuse to put these in the 'rude' category. I actually think it would be phenomenally stifling of useful thought and discussion in this world to categorically decrees boundary of rude expressions that aren't specifically directed at a particular person. I think that people should be able to say, in the right forum (and a personal blog is one) "I think XXX idea is totally asinine."
Otherwise, where precisely do you draw the line? If the phrase "People who believe in God are idiots" is rude, then what about "A belief in the supernatural requires an abandonment of rational thought, the very thing that makes us human, that makes us distinct from mere beasts". It is functionally the same sentiment (and if it isn't, then I'm sure we could come up with one that was more identical). What's the difference? That the first is more course? Less intellectual? The point is the same in both.
These are 'time and place' items - if you're in a zone where discussing the validity of religion, or a political ideology, or anything else is on the table, then it is on.the.table. Now, I think the first statement is immature and poorly reasoned and supported, and overall not a very useful addition to an idealogical discussion, but that's a different matter.
And I also disagree that we live in a binary world where things are either 'polite' or they are 'rude'. I think there is a whole universe of material in the world that is neither.
Most of the political spectrum, discussion, and analysis isn't particularly 'polite'. But I also think it's only rude when it stoops to personal attacks and direct mudslinging. "Polite conversation" usually can't even abide much discussion of politics, because it gets inherently outside the bounds of 'polite', but it doesn't always have to get 'rude'.