The two heresies were Adoptionism and Docetism. Hardly household words.
Adoptionism held that Jesus of Nazareth was born a human being and only became the Son of God at a later point in his life, by adoption, --usually at his Baptism--because he was righteous. Orthodoxy holds that he was both fully God's Son and fully human from the instant of his conception. Why is adoptionism cruel? Because it sends the message that grace is a reward for virtue --which sorta defeats the whole purpose of grace-- and thus lays the entire burden on poor humans to achieve our salvation. Orthodoxy's grace is a gift.
Docetism held that Jesus Christ was not actually a human being, a man, but only appeared to be. Because he was divine, it would be absurd and undignified to think that he could actually have a body and know passion or suffering. The cruelty here is to send the message that human beings as we actually are, embodied, cannot be saved. Orthodoxy's Incarnation holds that divinity and humanity are compatible.
If you put these two un-orthodox beliefs together, the message is that you are simultaneously a) incapable of becoming worthwhile because you are essentially worthless and b) entirely responsible for achieving your salvation.
A theological Catch-22.
Liberalism is similarly cruel. For the sake of its beloved victim minorities, it forces the majorities into situations where they are likely to fail to live up to the moral task set for them*, all the while telling them that it is their ingrained bigotry or prejudice or privilege which is to blame. And most of the victim minorities, who are likewise sold a bill of goods about what they should desire and see themselves deserving of, also fail: in the case of race, because of both incapacities and cultural variants and dysfunctions; and in the case of women --who have minority status if not minority numbers-- because achieving the feminist dream leaves so many of them unhappy.
And the majorities, which liberalism hates, are put in positions where they are in constant anxiety lest they should break any of the myriad rules of PC and then in a constant state of cognitive and emotional dissonance because what they experience is not what they are being told that they should experience.
Liberalism --like adoptionism and docetism-- hates human nature. Under the guise of ennobling it, it actually creates the conditions of its failure. To answer to which, of course, is always the same: more liberalism. Which is not only cruel, but crazy.
*I have likened this to the Catholic category of "occasions of sin", situations which, while not sinful in themselves, are likely to lead to sin. And should thus be avoided. My two favorite examples: the influx of Muslims and Africans into Europe, and the integration of females into the military. The first produces inter-group violence (and may lead to increasingly catastrophic forms of it). The second produces higher rates of rape and sexual abuse. In both cases, primary responsibility for these outcomes lies with the (liberal) decision-makers who put people into these situations, where the outcome --given human nature--could easily be predicted. They will, as always, blame negative results on "racism" and "sexism."