I’ve seen this one a bunch of times and it always irritates. First of all, it appears to be a fake quote. It’s attributed to the book Contact, but it does not appear in that book. Even if it did, it would be spoken by a character, not by Sagan. And I find it highly doubtful that he would say something this dumb:
And no matter who said it, it’s obviously false. The overwhelming majority of atheists used to be believers, so obviously they can be convinced. Not all of them, of course. Lots of believers are completely impervious to evidence or reason. But guess what? Lots of atheists are too. The kind of atheists who share bullshit like this thinking it’s profound or true.