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I was told by two respected Roshai Yeshiva that one who isn't certain of God's existence (an agnostic) is not deemed to be a kofer (denier of His existence) as long as he thinks that His existence is probable. So one who is 51% sure of God's existence is not a kofer, but one who is 49% sure of it is. They both admitted that they had no source for this, but thought it was logical.

Can anyone provide a source for this? Are there other views? Does one have to be 100% certain of God's existence to not be considered a kofer? Is someone who isn't convinced that God exists, but is a fully practicing Jew because of Pascal's wager considered a kofer?

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maybe the rosh yeshiva was just telling people to be content with simple faith and not to think too much due to this: judaism.stackexchange.com/a/5463/1857 – Gizbar Jan 30 '14 at 7:30
I don't think he was. One in particular is a very philosophical person who wouldn't say that. He told me that he was originally going to be a philosophy professor, but decided to become a Rosh Yeshiva instead. Either way, unless an agnostic view was permissible he wouldn't advocate relying on simple faith and living with doubts. – Ish Ploni ViKohen Jan 30 '14 at 18:28
Marpe Lenefesh commentary on ch.10 of Shaar Yichud: "As we find recorded in books, that most of the early philosophers became insane. And we see even in our generation - those groups which go after their opinions and investigations, either they became crazy or they go out to evil ways..." - not clear from here if actual apikores, but if you rely only on your intellect alone you will likely go off and become a full fledged bonafide apikores. – Gizbar Jan 30 '14 at 20:21
It’s difficult enough giving a p’sak in matters of hashkafah, and you want to know the shiur too? – J. C. Salomon Feb 5 '14 at 19:43
@IshPloniViKohen he was originally going to be a philosophy professor!?? Is that public knowledge? – Matt Oct 28 '14 at 18:27

The Rambam changes his language in two places where he discusses our awareness of G-d's existence.

In the introduction to the 10th chapter of Sanhedrin, where the Rambam lays out his 13 Principles, the Rambam discusses "belief" in Hashem's existence. (Depending which translation you look at, the term "belief" is in the text of the Principle itself, but in Kapach's translation it is not in the description of the Principle, but is in the Rambam's concluding paragraph, in which he says about all 13 "belief.") The Rambam (in Kapach's translation) sums up with:

וכאשר יהיו קיימים לאדם כל היסודות הללו ואמונתו בהם אמתית הרי הוא נכנס בכלל ישראל


"anyone who believes these 13 principles is not an apikorus.

On the other hand, in Yesodei Hatorah, both in the koseres and in the first halacha, the Rambam says that the mitzvah is to know that G-d exists:

יש בכללן עשר מצוות--שש מצוות עשה, וארבע מצוות לא תעשה; וזה הוא פרטן: (א) לידע שיש שם אלוה


The first Mitzvah is to know that G-d exists.

(In the Sefer Hamitzvos many printings have "להאמין" by the Mitzvah, but the mistake in that translation has been pointed out numerous times. See the notes in the Frankel Rambam there for a nice summary.)

R' Yaakov Weinberg explained that the discrepancy is because in the 13 Principles, the Rambam is not listing Mitzvos - he is making statements - if you believe these 13 things, you will not be an apikorus. For not being an apikorus, belief is enough. But the Mitzvah of awareness of G-d is to know that He exists - to take that belief and develop it into knowledge.

As an extra credit point, R' Weinberg explained why both sides of this contrast make sense. The Mitzvah has to be to know, because you cannot have a Mitzvah to believe in G-d - if you believe He exists, then the Mitzvah is unnecessary, and if you don't believe He exists, then you won't be accepting a commandment from someone you don't believe exists. The Principle is to believe, because the nature of the Principles is forming the framework in which you will not be able to justify for yourself to not serve Hashem. For that, belief that He exists is enough.

However, this just means that 100% is not the requirement to leave the category of apikorus. If someone is just playing the odds, but doesn't really believe that He exists, that would not suffice. They would need to have strong enough belief to not be able to convince themselves that He doesn't exist when it becomes convenient to do so. (I can't put a percentage on that #.)

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+1 and I'd add that R. Chaim Heller's notes to the first mitzvah in Sefer Hamitzvos writes that the Rambam uses a term in Arabic that is ambiguous, and can mean 'know' or 'believe'. The Frankel Rambam quotes him on this in their notes as well I believe. See also Moreh Nevuchim 1:50 – Matt Oct 28 '14 at 18:26
@Matt ya, R' Weinberg made mention of that. Although I thought the arguments in the Frankel notes that it is altogether mistaken were more compelling. Despite my complete lack of qualifications to really have an opinion on this. – Y ez Oct 28 '14 at 18:31
Same (re:qualification), but the use of the word in M.N. 1:50 sounds very much like what we'd call knowledge, but the most literal Hebrew and English translators (Shvartz and Pines respectively) have rendered it as 'belief' despite the context, which is why I'm inclined to think that they had reason for doing so. Shvartz even put the original Arabic word in parenthesis (a'athkad or something like that) to indicate its ambiguity – Matt Oct 28 '14 at 18:41
Is this Torah from R. Weinberg published anywhere? I really like it; I think it explains more than what's here – Matt Nov 28 '14 at 7:26
@Matt This specific point about לידע and להאמין is not in Faith and Fundamentals. I will check in Even Sh'sia if I remember. But I heard it on a recording. – Y ez Nov 30 '14 at 19:20

This Chabad Article deals with agnosticism which Thomas Huxley defined as the "doctrine that humans cannot know of the existence of anything beyond the phenomena of their experience" (and therefore, my words, cannot be sure of the existence of G-d).

The article quotes the sixth Lubavitcher Rebbe, Rabbi Yosef Yitzchak Schneersohn (1880-1950) who told one self-proclaimed atheist, "We are all believers in G-d. It is just a matter of definition."

Therefore someone who isn't 100% certain of God's existence should not be considered a unbeliever.

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Thank you for your answer. I think that most would agree that if one believes in the existence of God to the degree that he believes that George Washington was the first president he is considered a believer, even if he would concede that he cannot prove either one. My question is more in relation to one who has material doubts about God's existence. – Ish Ploni ViKohen Feb 9 '14 at 22:36

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