Perhaps Mr Heather is objecting to a certain lack of rigour in the statement; “probably” inclines to a more agnostic view, whilst as a predicate for relaxation the absence of God seems inane, if not vulgar. “There’s no God. Start reading Sartre” might be a better prescription. As a solution to existential angst, the buses aren’t really cutting it, but perhaps the real objection is that they reduce atheism to pugnacious sloganeering.
Dr Rowan Williams’s fearsomely impressive book on Dostoevsky suggests that some churchmen are willing to engage in rigorous intellectual debate on faith, but the best the atheists can muster is Christopher Hitchens thumbing his nose at a deity in whom he professes not to believe. Why bother? Discussion of secularism is already hidebound by political sensitivity, and whilst the buses aim to promote exactly such discussion, their cheerfully simplistic hectoring succeeds only in offending the faithful and the thoughtful alike.