Some time back I wrote this post.
Today, I came across a passage from a letter of Rav Kook that sheds light on the issue. He writes as follows:
"דע, שאע"פ שאיסור גמור וחולי רע
הוא, אפילו מי שמסתפק ומהרהר על דברי האמונה השלמה, מכל מקום לא מצינו שדנו
חז"ל דין אפיקורוס, כי אם על הכופר, דהיינו המחליט ההיפך"
(אגרות הראי"ה א, עמ' כ).
Big chiddush! Someone who is not sure if there is a G-d is doing a big sin and is terribly ill but doesn't have the halachic status of an Apikorus until he actively denies. So there are three levels: 1] Believers. 2] Deniers - who have the status of Apikorsim, and 3] Doubters [known as agnostics] - who are sinful and ill but not Apikorsim.
