Exactly the case. It’s a matter of indoctrination and blind acceptance. Faith versus knowledge.Yup, many atheists are former believers and they spent a lot of time working to understand the faith. Before I became an atheist I had spent 30+ years as a believer. My "path" to atheism was very detailed and took years of learning and reading and trying to understand.
For some believers they have a pretty limited grasp of their beliefs...they don't think too deeply about them and willfully ignore the difficult points. So atheists who know about the faith are the most terrifying.
And I can't really blame them. It's a perfectly human behavior to avoid those things which fundamentally mess with your view of how the entire universe operates. And in the case of religion, threats to that are threats to their immortal soul. OF COURSE they would hope that atheists don't understand.