The question that many ask on the Rambam is that one must believe in the entire Tanach and Chazal. So how can he limit our belief system to just 13 articles of faith?
Rav Chaim Soloveitchik answered that there is a difference between the 13 Principles and all of our other beliefs. If one doesn't know that Hashem made a bris with Avraham and promised him Eretz Yisrael he is not a kofer - even if he thinks that He promised Avraham Avinu the State of Texas. If one doesn't know that the Torah was given at Har Sinai but thinks mistakenly that it was given at the [let's say...] Young Israel of Woodmere - he is not a kofer - just an ignoramous. However, if one doesn't know one of the 13 Principles of faith he is a kofer - even if he has no bad intentions. He is not JUST an ignoramous because regarding the 13 Principles - ignorance is no excuse. One MUST know and believe in them in order to be considered part of Klal Yisrael and to go to Olam Haba. In the language of Rav Chaim ztz"l: א נעבעך א אפיקורוס איז אויך א אפיקורוס.
This is very difficult for a number reasons:
The first is that this excludes many many good Jews from Olam Haba. A person can learn in Yeshiva for years and not know that Hashem is the absolute metzius and that only He MUST exist [Principle #1] or the notion of Yichud Hashem. As a matter of fact - go up to the average guy and ask him what "Yichud Hashem" means. It is likely that he never heard of it and/or gave it thought. Also, what about all of the Nashim Tzidkaniyos throughout the generations? They knew/know all 13 Principles?? Unlikely. Do we sit with Bar Mitzvah boys and teach it to them right before their Bar Mitzvah? According to Rav Chaim - we really should because they could be walking Apikorsim for years.
To make things worse, the 13 Principles printed in our siddurim are a very inaccurate portrayal of what the Rambam actually said, as we have been demonstrating in our recent posts. And who on earth learns the Rambam's Hakdama to Chelek where he explicated the 13 Principles? Go to the Mir and Lakewood and take a poll. And if they haven't - what about the average "baalebos"?
Also, what are we to do with the following Rambam [Mamrim 3/2-3]:
במה דברים אמורים באיש שכפר בתורה שבעל פה במחשבתו ובדברים שנראו לו והלך אחר דעתו הקלה ואחר שרירות לבו וכופר בתורה שבעל פה תחילה כצדוק ובייתוס וכן כל התועים אחריו אבל בני התועים האלה ובני בניהם שהדיחו אותם אבותם ונולדו בין הקראים וגדלו אותם על דעתם הרי הוא כתינוק שנשבה ביניהם וגדלוהו ואינו זריז לאחוז בדרכי המצות שהרי הוא כאנוס ואע"פ ששמע אח"כ [שהוא יהודי וראה היהודים ודתם הרי הוא כאנוס שהרי גדלוהו על טעותם] כך אלו שאמרנו האוחזים בדרכי אבותם הקראים שטעו לפיכך ראוי להחזירן בתשובה ולמשכם בדברי שלום עד שיחזרו לאיתן התורה [ולא ימהר אדם להורגן].
In other words - the Rambam seems to be very understanding and accepting of people who don't know any better.
Rav Aharon Soloveitchik answered the question on his grandfather as follows:
Moreover, when the Rambam writes in Hilchos Teshuva that one must believe that Hashem has no body, the Raavad comments that people greater than the Rambam made that mistake - implying that they are not Apikorsim. Some even say that the Rambam would agree with the Raavad and that it is not a machlokes.
The Lubavitcher Rebbe in a brief letter argues with Rav Chaim and says that if one is not a purposeful Apikorus he can't be considered an Apikorus. Rav Chaim Hirshonzohn [Malki Bakodesh Vol. 2 who lived in the late 1800's and early 1900's as did Rav Chaim] also understood that if one is an Apikorus Bi-shogeg he remains within the fold.
As they say in the classics - ועוד צריך עיון!!!