The original slogan read, "There is no God". Unfortunately, Britain's advertising laws forced them to add the "probably" in there, to avoid offending anyone. Plus, being an atheist doesn't mean that you're absolutely, 100%, no shadow of a doubt certain that there isn't a God, it just means that you don't believe in one and act on the assumption that he doesn't exist.
And it's not like every atheist in the world is involved in that campaign... so it's kind of dumb to judge all of us based on something that only a few people are doing.
Finally, "probably won't" is the polar opposite of "probably will", so shut the fuck up.