During the last couple of centuries, the following pattern has repeated itself many times.
1. A few insightful individuals recognize that a socially- and religiously-endorsed practice is actually immoral.
2. They speak out.
3. Awareness gradually spreads until almost everyone sees the immorality of the practice.
4. The practice is widely discouraged, then banned, and eventually more-or-less halted.
There are lots of examples: Slavery ended; women voted, interracial dating was legalized, sexual harassment was criminalized, "separate but equal" was rejected, social security was accepted, poor people got food stamps, gays got rights, ...
Each time, conservatives fought to retain (conserve) the practice. They apocalyptically warned that crucial social institutions would crumble if the practice ended. Eventually, in each case liberals won and the sky did not fall. Indeed, the world became a bit better, morally speaking. This is what moral progress looks like.
This pattern raises a question, a concern, and a hope.
(A) We look back into the past, and wonder how earlier generations could have missed the immorality of practices which now appear blatantly immoral.
(B) We look forward into the future, and worry that we might be blind to the immorality of some ongoing practices.
(C) We look around in the present, and wish we knew how to convince others that certain currently-controversial practices are actually immoral.
Abraham's near-sacrifice of Isaac (Gen 22) is one of the first recorded instances of an individual's recognition that a socially- and religiously-sanctioned practice was immoral. Properly interpreted, this story will dispel the wonder, alleviate the worry, and grant the wish.
The binding of Isaac
To understand the story of Abraham's near-sacrifice of Isaac, one must appreciate that child-sacrifice was widespread in the ancient Near East. So when Abraham set out to sacrifice Isaac, he was doing nothing unusual. He was following a socially- and religiously-sanctioned practice of his day. Every parent expected to sacrifice a child; every child worried that they might be killed on an altar. I imagine a tremor of terror in Isaac's voice as he asked,
Here are the firestone and the wood; but where is the sheep for the burnt offering? (Gen 22:7)
Since child-sacrifice was common and expected, Abraham's initial willingness to sacrifice Isaac was not a groundbreaking act of faith. If we celebrate him for his willingness to sacrifice Isaac, we must celebrate all of other parents who were likewise willing. Worse yet, we will be celebrating him for being a morally blind conformist, uncritically accepting a horribly immoral practice.
There is nothing admirable about a willingness to murder children, even when ordered to do so by God. God had already announced a prohibition of murder, both to Cain and to Noah (Gen 4:10-12, 9:5-6). Abraham had already articulated the principle that killing innocent people is wrong, and demanded that even God obey this principle (Gen 18:23-25). An interpretation which praises Abraham for a willingness to murder is prima facie problematic.
I suggest an alternative interpretation. Let us celebrate Abraham's ultimate unwillingness to sacrifice Isaac. The fact that he does not kill Isaac, rather than his initial acquiescence, is what sets Abraham apart from, and above everyone else in his society. It is that refusal which makes him a visionary moral revolutionary worthy of our respect. Moreover, Abraham's descendants reject child sacrifice. This suggests that Abraham not only saw child-sacrifice as immoral, he managed to end the practice.
Of course, there is a textual problem. An angel praises Abraham for his willingness to sacrifice Isaac (Gen 22:12, 15-18). How can Abraham be seen as a refusenik?
Perhaps the angel was an afterthought or metaphor for conscience, added after Abraham changed his own mind so that Abraham could tell everyone else that God was on-board with ending the practice of child-sacrifice. Admittedly, this interpretation requires some exegetical jujitsu, but it has the advantage of making Abraham admirable, while the alternative is to applaud him for being willing to perform a morally monstrous act just like everyone else.
Under my interpretation, the story of Abraham's near-sacrifice of Isaac suggests answers to the three questions above.
(A) How do people manage to miss the immorality of blatantly immoral practices?
Some of the practices are secret (or "open secrets"); they are hidden by privacy, darkness, a conspiracy of silence, etc. For example, violence and discrimination against Blacks by police has been rumored for years, but has been surprisingly unstudied. It has taken widespread video clips of police prejudice to bring it into the open where none may deny it.
Moreover, accepting damning truths about one's own society and/or religion is painful, and people avoid painful things. Abraham is a moral superhero, but even he could not see past his culture's assumptions until the last minute. This aspect of the story enjoins us to acknowledge the difficulty, to cut people reluctant to recognize the immorality of a practice some slack.
(B) How can we recognize an immoral practice that is not already so labeled?
Pro tip: If some group of people seems to be suffering (maybe even dying) because of a practice, that is a red flag. But how can we be sure?
Again the story of Abraham's near-sacrifice of Isaac is helpful, this time by providing a useful exercise. God does not ask Abraham to kill a random person. God emphasizes the stakes by the wording of his command, "Take your son, your favored one, Isaac, whom you love..." (Gen 22:2). Not only does God demand the death of a loved one, since God had promised to make Abraham a great nation specifically through Isaac (Gen 17:19), to sacrifice Isaac Abraham must relinquish his faith that God's promises are trustworthy.
When wondering about a red-flagged practice, try imagining yourself in Abraham's position. Imagine that one of your loved ones is the target of the practice, and imagine that your faith in some fundamental institution is in question. For example, suppose that you are White, and your faith in the fairness of the American justice system is one of your bedrock beliefs. Now imagine that your child is Black and stopped for "speeding." Does this thought experiment make the immorality of certain police practices stand out?
(C) How can we convince others that a practice is immoral?
I suggested above that the angel's role in the story is to enable Abraham to convince others that God no longer required child-sacrifice. If Abraham had announced to everyone, "I now see that child-sacrifice is wrong," he would have gotten nowhere. But I imagine that when he said, "An angel told me that child-sacrifice is no longer necessary," the practice stopped.
The story shows that one way to eliminate a practice is to convince people that the practice conflicts with the core values of their society and/or religion. In Abraham's day, the core values were expressed as God's orders; today the core consists of fundamental principles. For example, the point of saying that Black lives matter is to appeal to the fundamental principle of our society that all people have the right to life, and the fundamental principle of mainstream religions that all human life is sacred.