The possibility of understanding Sacred Texts is established within Theistic traditions by the will of God who reveals them. In the atheistic Mīmāṃsā, on the other hand, it is based on our linguistic expertise: we have to rely on worldly meanings of words even while reading Sacred Texts since, else, we would not have any key as to how to interpret them. Hence, the mastery of worldly meanings is a pre-condition for the understanding of a Sacred Text. But what if that texts prescribes a kind of duty which is fully new (apūrva), un-preceded, that is, non-attainable through any other (worldly) kind of knowledge? Should not it remain beyond any possible grasp?
More in general, the issue here sketched raises thought-provoking questions for all theological discourse. How can, in fact, the non-human be expressed in terms accessible to human beings?
However, let me situate the problem within the school I know better, Mīmāṃsā.
According to both the Bhāṭṭa and the Prābhākara school of Mīmāṃsā, the relation betweeen a word and the entity it means is perpetual (nitya). Nonetheless, this does not amount to say that everyone, upon hearing for the first time a word, automatically understands its meaning. Rather, one needs first to acquire proficiency in language use through the usage of elder people and through the ensuing activities (both these aspects may be referred to as vyavahāra). E.g., after having heard one's grandfather ordering:“Bring [the] cow!," one sees one's father bringing a cow. Through many similar instances, one eventually learns the meaning of the words “Bring!” and “cow”.
But, according to the Prābhākara, the meaning conveyed by the Veda is a duty (kārya) which is unprecedented (apūrva). Hence, how could it be possible to learn the relation between a word and a meaning such as the unprecedented duty through the usage of the seniors? And if this is not possible, how could one understand the meaning of the Vedic words referring to it? In fact, though the relation between Vedic words and the unprecedented duty is fixed, a meaning can be grasped only by people who have previously understood, by means of the linguistic usage of senior speakers, its relation with the word signifiying it. Nor can it be said that one can learn the meaning of Vedic words referring to an unprecedented duty through the Veda itself, as in this case there would be a vicious circle (the elders' usage would depend on the Veda, whose understanding depends on the elders' usage).