Those who don't dive deeper into understanding Epicurus' precise views of things are likely to be taken aback by the fact (reported by Philodemus) that Epicurus "railed against" and "questioned the very sanity" of atheists. (Quoted by Philodemus, Piet. col. 19, Obbink 1996, 142-143.
Many readers of Epicurus are atheists and don't like this position, and they are even willing to resort to the argument that Epicurus was "playing it safe" so as to avoid the fate of Socrates and Anaxagorus. I categorically reject that explanation and think it's insulting to Epicurus and to anyone who takes Epicurus seriously.
But it's also on its face insufficient to say that Epicurus held that "there are gods because everyone things there are." What is meant by "gods" and what is meant by "everyone thinks there are."
I'm starting this thread so we'll have a prominent place to collect references and arguments as to why, from Epicurus' point of view, those who flatly and totally deny the existence of all gods are essentially insane.
And if they are insane, in what constitutes sanity?
Although it doesn't focus on this issue there is a lot of good background information about this topic in David Sedley's "The Atheist Underground", which is what prompted this thread now.
Of course Epicurus' larger position on the nature of divinity is explained in greatest length by Velleius in Cicero's "On the Nature of The Gods," so pending further discussion that's the best source of the answer.