"'Go back no more than four hundred years or so. Over all the time since, Christendom has been a tyranny of a rare sort. By way of the soul it rules the minds of most and the acts of all. As effect, no wars throughout Europe but the one, a war with long breaks of peace, a war against a power that can never be crushed and can be held in only by standing in arms from year to year: the best possible form to draw off any will to rebel or quarrel. And, in the last fifty years, Christendom has finally drubbed a power much more awful than the Turk could ever be, one that now lives on as it can in New England among boors and savages: science. God be praised.'
"'Amen,' said Lyall automatically.
"'Amen to amen. It was a close thing. A little longer, and science would have abolished God and brought our world to ruin.'
"'You don't mean abolish, you mean take attention from, leave on one side.'
"'I mean abolish, I mean deny, I mean disprove.'"
In 'The Alteration', Kingsley Amis explores what the world would have happened if the influence of Catholicism had never declined. In this it echoes Keith Roberts' superlative 'Pavane', one of a number of alternate history novels that Amis tips his hat to. As in 'Pavane', Amis' story focuses on the characters in this world, allowing the way different people interact to tell us about how it's different from our own instead of relying too much on heavy exposition. This allows Amis to create a well realised alternate time line, with many historical in-jokes and nods to other works in the same field, whilst still telling a powerful and satisfying story.
In this case, the jonbar point is the Reformation. In the world of 'The Alteration', instead of splitting with the Catholic church, in order to prevent a schism Martin Luther becomes pope. Protestantism never emerges, and so the Catholic influence over Europe never diminishes but only grows stronger. The world described in 'The Alteration' is similar in a number of respects to that in Roberts' 'Pavane'. In both books Catholicism informs and controls all aspect of life in Britain. Science is outlawed as heresy, so electricity is banned and most technology runs on steam or gas. The world wars never happened; in 'Pavane' Roberts puts this down to the lack of technological development, in 'The Alteration' one of the priests explicitly states that peace within Catholic Europe was achieved by uniting them in war against the Turks. Both books explore a society in which the power of the patriarchy is maintained by religious oppression, where dissent is swiftly and brutally punished. However the concluding tone of the books is a little different. Whereas Roberts ways up the cost of religious oppression against the technological horrors of the holocaust and nuclear destruction and finds merit in a world developing science more slowly and cautiously, Amis' book is much more cynical and dystopic. At the end of 'The Alteration', it's revealed that the pope, unwilling to change the church's stance on birth control, is trying to solve the population growth problem across Europe by secretly putting drugs in the water to reduce the birth rate, a plan he is testing out on unknowing subjects.
'The Alteration' focuses on Hubert Anvil, a choirboy with an incredible voice who is marked for castration by the church in order to preserve his voice for the pope and the glory of God. Amis uses this central idea as both an example of the unnecessary brutality of an institution that would mutilate a child, and as a way of illustrating how much of the Catholic church's control over its people comes from control of sexuality. As a New England reverend says in the book,
"What they had intended to do to little Hubert Anvil was shocking without being surprising, considered Pellew. All their temporal over-magnificence, all their pharisaism, all their equivocation, all their ruthlessness came from one source: the celibacy of their priesthood."
Not only is Hubert denied from ever reaching sexual maturity, but priests are forced to be celibate, birth control is banned, and homosexuality, indeed any kind of sexuality outside of married heterosexuality, is banned. Amis explores the importance of sexuality in the human experience, the part sexuality plays in one's self identity, and hence how repressing this becomes a powerful tool of social control.
Resigned to his fate but wanting to know what it is he will be missing out on, Hubert asks his family and friends what sex is like. This allows Amis to explore the complexity of human sexuality, and the confusion and mystification that surrounds it for young adolescents. From the stilted allusions from his conservative father's answer to the more down to earth answer from his brother to the crude smut he hears from his classmates and the stable boy, to his own budding feelings for a girl, Hubert is unable to collate these answers into a consistent whole. Amis deftly portrays the curiosity, excitement, apprehension and fear that adolescents face around the idea of sex.
While it's realistic that Hubert would not feel comfortable asking friends who are girls or his mother about sex, this does mean that the book builds up a pretty one-sided, male-centric view of sex, and only heterosex at that. While you can argue that the book sidelines women and female views of sexuality, and any view whatsoever of LGBT sexuality, as an artistic choice to reflect the deeply conservative society it depicts, that doesn't mean these people wouldn't exist or have a viewpoint, albeit one they would have to keep secret, in such a society. At least Hubert's mother, Dame Anvil, is a strong enough character to resist Hubert's alteration as much as she can, and is given sexuality, though she is reduced to using it to protect her son. It's also worth pointing out that the elder castrati who select Hubert for alteration are very othered, in some part to make Hubert's fate even more awful than it already is, and again this is something that would realistically be a thing in the society depicted, but it's still uncomfortable.
Ultimately Hubert decides that something this important must be worth keeping, even with the promise of wealth and celebrity weighing against it, and tries to escape to New England, where Protestantism has belatedly emerged. He gets as far as the airship just before takeoff before coming down with testicular torsion and having to have his nads surgically removed anyway. This is a particularly cruel diabolus ex machina on behalf of the author, but it only serves to emphasise the unstoppable power of the Catholic church in this world: an unlucky coincidence starts to look like God really is on their side.