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[Q4]: Is the fitrah considered a proof against those who disbelieve? [A4]: The proof is established by way of sending the messengers.  As for the fitrah alone, then it is not sufficient as a proof (against someone).  If the fitrah was a proof, then Allaah would not have sent the messengers: "Messengers, conveyors of glad tidings and warners, so that the people would have no proof against Allaah after the (sending of the) messengers." [4:165] You do not know what is obligatory, prohibited, or disliked (from the fitrah), these are things that only a messenger can clarify.  Rather, the fitrah is something that makes people inclined towards good.  It is not sufficient by itself.  If a person lived upon the fitrah alone and did not learn or do anything, then this is not sufficient.