There are many criticisms of sentimentalist theories like Hume’s, and many were pointed out by Kant. One of these mentioned above: variation in moral feeling means that it cannot provide an adequate standard of good and evil. Also, Kant thought that if you based moral obligation in feeling you would get into the awkward position where emotional people would end up with greater obligations than hard-hearted people. These and other criticisms led Kant to a moral theory that is starkly different to Hume’s when it comes to the role of reason and the passions in morality.
For Kant, the only thing that can have proper moral value is a good will. The will is what makes us act, and it can be influenced by lots of different things: desires, passions, instincts, and by reason. Kant thinks that we can only grasp moral laws, and the obligation these laws lay on us to obey them, through reason. The will is only thoroughly good when it acts solely because it recognises these obligations reason discovers, that is, as Kant puts it, out of duty. I might keep a promise – an action that is prescribed by the moral law – because I feel like it, or because of my affection for someone. But if inclination or affection is what determines my will, then there is actually less moral value in my action, than if I had not wanted to do it, and yet had still done it out of duty. The will is only thoroughly good when it is respect for the moral law – respect for what’s right – that makes me act.
Another important feature of Kant’s moral theory is that he thinks that the will is autonomous. By this he means, at least, that it can be determined by reason alone, rather than always caused by instincts or desires as an animal’s will is. In fact this is what makes us capable of moral action. Reason provides the moral law, and the will (at least the good will) obligates itself to follow it. So Kant sees people as self-legislating: morality is grounded in one’s own reason and will.
The supreme moral law Kant thinks reason gives us is a categorical imperative: an imperative is just another name for a command, and it’s categorical because it applies to all rational creatures, all the time, regardless of their particular situation. (Categorical here means unconditional or absolute, as in My answer is a categorical ‘no’.) A categorical imperative is a principle for acting that meets a certain criterion, namely, you could will that it become a law for everyone. If you could, then acting in accord with that principle is right (and failing to is wrong). So if your principle was promises should be kept, you would need to think about whether you could want everyone to always act that way, and if you would, then promise keeping is part of the moral law, and obligatory.
It is this unconditionality that Isabel reacts against when she thinks about how Kant would approach the question of whether or not gambling is wrong. Isabel herself thinks it would probably depend on your particular circumstances. Gambling when you don’t have enough money to feed your children is clearly wrong, but what about gambling with your surplus income. Is it okay then? Isabel is open to the possibility, but Kantians would not, and “that was the problem with Kantian morality: it was so utterly predictable, and left no room for subtlety” (p. 187). In fact reading SPC, you get the feeling that Isabel really doesn’t think much of Kant’s moral philosophy: earlier in the novel she keeps a paper that criticises Kant till last so she can “savour” it (p. 92).
It’s hard to cover a theory as complex as Kant’s in a few paragraphs, and all I’ve really given here is a taste. Kant is a notoriously difficult philosopher to understand, and his moral theory comes with all the baggage of his other philosophical views (his metaphysics, epistemology & ontology). I don’t pretend to understand all of it, so it’s hard for me to evaluate whether you could consistently be a Christian and a Kantian. Kant himself claimed to be a Christian, so if you’re happy to take his word for it, that could answer the question! (In fact, while Kant rejected all the usual arguments for the existence of God, he has his own, which runs along the lines that morality is only possible, in practice, if there is a God to hold people accountable for their actions.) The categorical imperative, like Hume’s sympathy, is evocative of the Golden Rule which Christians know and love, though deliberately less personal. Kant at times says the moral laws are divine commands, but his view of the relation between God, us, and the moral law is far from the traditional view! The idea of autonomy is something Christians sometimes have a knee-jerk reaction to, as though it were always a bad thing indicating a lack of submission to God, but this isn’t necessarily the case. You need to ask what the autonomy is from to decide whether it’s good or bad. It was the Reformers, after all, who fought for autonomy from the Catholic church, and it’s part of the job of parents to develop their children’s autonomy so that they become independent adults. I don’t think I have a good enough grasp of Kant’s view of the autonomy of the will to state categorically (!) whether it’s opposed to Christian doctrine or not. I wouldn’t rule it out, but I’m not entirely comfortable with it either. I also have qualms about the value placed on doing your duty on purely rational grounds. It seems rather cold and hard by comparison with the compassion and affection we see Jesus acting with. I think most of us would find counterintuitive to factor emotion out of the moral equation, but at the same time, you can see Kant’s point about the goodness of doing what’s right even when your desires are telling you otherwise: think of Jesus on the cross. The best I can do is this: I’m not going to say you couldn’t be a Christian and a Kantian, but if you were a Kantian you wouldn’t be an orthodox Christian!
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