Morality comes from scripture. For starters, here is what Adi Shankaracharya says in this section of his Brahma Sutra Bhashya, which is actually about the subject of animal sacrifice:
[The reasoning criticizing animal sacrifice] is not valid, because our knowledge of what is duty and the contrary of duty depends entirely on scripture. The knowledge of one action being right and another wrong is based on scripture only; for it lies out of the cognizance of the senses, and there moreover is, in the case of right and wrong, an entire want of binding rules as to place, time, and occasion. What in one place, at one time, on one occasion is performed as a right action, is a wrong action in another place, at another time, on another occasion; none therefore can know, without scripture, what is either right or wrong.
For a more systematic look at things, we must turn not to the Brahma Sutras, which begin with "Athato Brahma Jijnasa" or "Now therefore there is a desire to know Brahman", but to the Purva Mimamsa Sutras, which begin with "Athato Dharma Jijnasa" or "Now therefore there is a desire to know Dharma". Here is what Adhyaya 1 Pada 1 Sutra 4 of the Purva Mimamsa Sutras says:
satsaṃprayoge puruṣasyendriyāṇāṃ buddhijanma tatpratyakṣamanimitrta vidyamānopalambhanatvāt
That cognition by a person which appears when there is contact of the sense-organs is "sense-perception", and it is not a means (of knowing Dharma), as it apprehends only things existing at the present time.
And here is what Shabaraswami's commentary says about this Sutra:
The examination (promised in the preceding Sutra) is as follows:- Sense. perception is not the means (of knowing Dharma), - why?- because the character of Sense-perception is that it is "that cognition by a person, etc."(sutra); that is, it is that cognition which a man has when his sense-organs are in contact with the object cognised. - Dharma however is something that is yet to come, and it does not exist at the time that it is to be known; - while Sense-perception is the apprehending of an object that is actually present and not non-existent at the time (of cognition); - hence Sense-perception cannot be the means (of knowing Dharma). In the Sutra, no stress is meant to be laid upon either "cognition", or the "appearance", or upon mere "contact"; the only factor meant to be emphasised is the fact of its being such as is possible only when there is contact between the sense-organ and the object, and not when there is no such contact between them. If stress were laid upon several factors, then there would be syntactical split. As for (the other means of Cognition.) Inference, Analogy, and Apparent Inconsistency, these also presuppose (are based upon) Sense-perception; hence these also cannot be the means (of knowing Dharma). Nor can Dharma be amenable to “Negation' (i.e. it cannot be regarded as non-existent; because of the reason given in the next Sutra which indicates the real means of knowing Dharma).
The idea is that perception and inference can only tell us about the world of things which already exist. They cannot tell us about that which does not yet exist, namely the consequences that you will experience for the actions that you do. So we require some other means of knowledge to tell us what actions are linked to what kinds of consequence. What is that means of knowledge? It is Hindu scripture. Here is what Adhyaya 1 Pada Sutra 5 of the Purva Mimamsa Sutras says:
autpattikastu śabdasyārthena sambandhastasya jñānamupadeśo 'vyatirekaścārthe 'nupalabdhe tatpramāṇaṃ bādarāyaṇasyānapekṣatvāt
The relation of the word with its denotation is inborn.- [Vedic] instruction is the means of knowing it (Dharma) - infallible regarding all that is imperceptible; it is a valid means of knowledge, as it is independent, according to Badarayana.
The idea is that the human mind is powerless to find out what is right or wrong, since as I discussed above morality concerns that which does not yet exist, but the Vedas, since they are Apaurusheya or authorless, have the capacity to talk about the imperceptible realm, since they do not depend on the knowledge of a human being. (The authoritativeness of Smriti is derived from the authoritativeness of the Vedas in Adhyaya 1 Pada 3 of the Purva Mimamsa Sutras.)
Now I could go on to discuss Shabara's commentary on Adhyaya 1 Pada 1 Sutra 5 and on the subsequent Sutras of Adhyaya 1, which demonstrate the Pramanya (authoritativeness) and Apaurusheyatva (authorlesness) of the Vedas, but that would require me to write a book. (Which I might well do! See idea #1 in my Medium post here.)
Bottom line, people might have all kinds of false, ever-changing beliefs about morality due to the temptations of the Kali Yuga, but what is actually moral and immoral is based on scripture, not on human beliefs.
