The tl;dr is that Eagleton 'believes' in evil, and so sets his face against those (as he sees them) modish and misguided leftists and postmodernists who think morality all relative, evil just another name for 'anti-social activity' and so on. I quite liked this, but wished Terry had made his case with fewer Massive Gaping Holes in his argument. For example, Eagleton believes that one must actively accede to evil for it actually to be evil:
To be damned, you must know what it is you are turning down, rather as you must be of sound mind to be married. ... It would be unforgiveably absentminded of the Almighty to pack off some of his creatures to everlasting torment without having alerted them to the disagreeable possibility in the first place. You cannot end up in hell by accident, any more than you can learn Portuguese by accident. Arguing by analogy is a notoriously foggy process, and best avoided, but even so this is spectacularly poorly chosen. I mean really: how does Eagleton think Portuguese babies learn Portuguese? Does he picture them sitting down, in nappies, with textbooks of Portuguese grammar in front of them and getting stuck in? Of course not. They pick it up as they go along; they absorb it from their environment whilst they're doing other things like playing and eating and gurgling. They learn their language inadvertently, which is another way of saying accidentally. And, to work back along the analogy, there are surely people brought up in environments that acculturate them to a range of evil beliefs and actions, from sexism and racism to active cruelty and oppression, in ways that ramp up so incrementally and slowly that at no point are they actively aware this is what is happening to them. Might this not be ... well, most people, actually? (Of course, what Eagleton means that I, native English speaker, nearly 50 years of age, am not going to learn Portuguese 'by accident'. But neither, not having been brought up in the Hitler Youth by Nazi parents, and osmotically taught the natural inferiority of Jews, am I suddenly going to turn around and murder Semites).
There's a real blind spot in the book, here. Eagleton seems to cleave to what one might think a rather unMarxist view that Providence (since he doesn't bring God into it) always arranges a moment of moral clarity for every adult individual at some crucial moment, the Rubicon flowing calmly before them, enabling them to choose to do good or evil.
You can believe in evil without supposing that it is supernatural in origin. Ideas of evil do not have to posit a cloven-hoofed Satan. It is true that some liberals and humanists deny the existence of evil. This is largely because they regard the word 'evil' as a device for demonising those who are really nothing more than socially unfortunate. It is what one might call the community-worker theory of morality. It is true that this is one of the world's most priggish uses, as we have seen already. But to reject the idea of evil for this reason works better if you are thinking of unemployed council-estate heroin addicts rather than serial killers of the Nazi SS. It is hard to see the SS as merely unfortunate. One should be careful not to let the Khmer Rouge off the same hook on which delinquent teenagers are impaled. Since a good proportion of the Khmer Rouge were precisely delinquent teenagers (kids, basically, raised, acculturated and pressured into believing that they were serving a higher good, and liable to be killed if they didn't go along) this looks almost like Eagleton is trolling. I daresay some SS officers had half-hour Long Hard Thinks, when they knew themselves to be acting in an evil way, balanced this against the option of doing good, and rejected the latter. But surely not many.
And this is the issue. If you're living in Nazi Germany, fighting evil like Dietrich Bonhoeffer even at the cost of your own life is one option. But to do so means setting your individual opinion over the opinion of the crowd, the people, the nation. Such martyrdom is sometimes necessary. But it rather contradicts Eagleton's view, expressed in a lengthy reading of Pincher Martin, that 'like Faust, the damned are too proud to submit ... they will not bow the knee to the finite, least of all to their own creatureliness This is why pride is the characteristic Satanic vice' . Because 'pride' has a number of quite separate valences. It can be bad egoism, of course. But it can be something else.