We, collectively (there is really no such thing, but most people seem to assume there is), do not learn new things about what is good and what is not good, what is right and what is wrong. Fashions in ideas, in morality, change, they evolve in the zoological sense, but they do not progress in the political sense, they do not move towards any particular goal (of enlightenment). Today we (that is, those of us who live in a certain society, or kind of society) are told that we must accept, or that we do accept, certain ideas as true and good, that those who do not accept them are unpersons, to be despised as insufficiently transcendent in their goodness.
England can be a bewildering place at times. You can be all but thrown out of polite society a club for not condemning vehemently something which nobody would have thought twice about the previous week. Politicians, broadcasters, people whose existence is noticed in the media or who have a position of responsibility, can find themselves condemned, pilloried, ostracised and summarily dismissed because they expressed an opinion or just used a word that someone else was able to take advantage of. Politics, even at the everyday social level, can be a nasty business, and what is right today can be made wrong tomorrow, retrospectively, by the use of the power of the voice.
In these cases specific ideas of right and wrong are deliberately changed, usually for bad reasons, but there is also a great deal of the social equivalent of genetic drift involved. Changes come about by statistical perturbation, and are incorporated into, or rejected by, the individual’s moral framework, and so become a part of the social morality against which we will be judged, and to some extent judge ourselves.
It is hardly surprising that religions can accumulate enormous temporal power, since they offer eternal life, strong personal identity and moral certainty. These are pillars of our psychological strength, because biologically we are social animals and consciousness of our own mortality requires us to reason our way out of the dead end.
Thus religion offers much more stable ideas of right and wrong than normal social exchange, because while the latter is highly fluid and has no meaningful reference points other than its own immediately prior form, the former takes an external and timeless reference and keeps track of itself over periods as long as possible or useful. Social morality, even where it is used to gain control and power, has no real purpose except synchronic cohesion.
It doesn't matter if it changes with time, as long as there is a single recognisable form at any particular moment.
We are used, historically, to the idea that people from different countries, especially those with sharply contrasting cultures, will want to murder us. We don’t even bother to think of it as wrong. But those who murder or mutilate their own children are in quite a different category. In general those who slice off their daughter’s clitoris, sew up her vulva, or murder her for ‘company-keeping’ as Doucie Davie Deans calls it in the Heart of Midlothian, do so because they believe that what they are doing is right. There is no point trying to explain why we believe these things are wrong. We can only insist that they are wrong, act in consequence, and wait for the idea to be slowly accepted. Those who are closest to us, in other words, must be morally assimilated. That is, after all, who concepts of morality have always changed.