The Rambam says that one of the categories of a 'min' is someone "who says there is no God and the world has no leader"
Does it have to be him actually saying it, or is just believing that sufficient? If the latter, is there a minimum time period of such belief that will qualify him as a min?
For example, if one has temporary doubts about G-d's existence [so belief, followed by brief moments of doubt/non belief, followed by belief again], what is his status?
In Hilchos Teshuvah 3:14 Rambam states that even if one denied God's existence his entire life, he can still repent before he dies and get Olam Haba.
במה דברים אמורים שכל אחד מאלו אין לו חלק לעוה"ב כשמת בלא תשובה אבל אם שב מרשעו ומת והוא בעל תשובה הרי זה מבני העולם הבא שאין לך דבר שעומד בפני התשובה אפילו כפר בעיקר כל ימיו ובאחרונה שב יש לו חלק לעולם הבא שנאמר שלום שלום לרחוק ולקרוב אמר ה' ורפאתיו כל הרשעים והמומרים וכיוצא בהן שחזרו בתשובה בין בגלוי בין במטמוניות מקבלין אותן שנאמר שובו בנים שובבים אע"פ שעדין שובב הוא שהרי בסתר שב ולא בגלוי מקבלין אותו בתשובה
When does the statement that these individuals do not have a portion in the world to come apply? When they die without having repented. However, if such a person repents from his wicked deeds and dies as a Baal-Teshuvah, he will merit the world to come, for nothing can stand in the way of Teshuvah.
Even if he denies God's existence throughout his life and repents in his final moments, he merits a portion in the world to come as implied by [Isaiah 57:19] "'Peace, peace, to the distant and the near,' declares God. 'I will heal him.'"
Any wicked person, apostate, or the like, who repents, whether in an open, revealed manner or in private, will be accepted as implied by [Jeremiah 3:22] "Return, faithless children." [We may infer] that even if one is still faithless, as obvious from the fact that he repents in private and not in public, his Teshuvah will be accepted. (Chabad.org translation, my emphasis.)
