I am not at all sure most Mormons know what they are supposed to believe.
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Part of that is the complete lack of systematic theology in Mormonism. Thier theology is haphazard which makes it easier to keep control on its members (harder to see the contradictions) and easier to dismiss its critics (”we don’t believe that anymore” cry even though many members do)
The things listed in the OP are very common teachings and would be known by all members.
How do they find OUT??
Official sites are sites supported by LDS officials unless said official sites are considered unofficial by said officials.At that point such sites are unofficial unless officially referenced for official purposes by officials who can do so officially.This should not be misconstrued as an indication that official sites can be unofficially recognized as official nor should it be implied that unofficial sites cannot contain official information, but are not officially allowed to be offical despite their official contents due the their unofficialness.Official sites will be official and recognized as official by officials of the LDS unless there is an official reason to mark them as unofficial either temporally or permanently, which would make the official content officially unofficial.This is also not to imply that recognized sites, often used on FR by haters and bigots cannot contain official information, it just means that content, despite its official status, is no longer official and should be consider unofficial despite the same information being official on an official site elsewhere.Even then the officialness my be amended due to the use of the unofficial information which may determine the officialness of anything be it official or unofficial depending on how and where it is used officially or unofficially.