You do raise a good point Pianodwarf that objective morality is not a perspective that only exists for God-botherers, whilst Kant was a God-botherer himself, his basis for moral was not derived from scripture. We studied Kant's Categorical Imperative when I studied philosophy and whilst there are many things Kant wrote I found myself at tune with. However, I did not acquire his views on morality. Whilst I can appreciate how the Categorical Imperative works and I think to a degree it is a sensible approach as far as absolutism goes. But it still, I believe, suffers from what any system of moral objectivity suffers from is that a situation may surpass whether those object morals can apply.
Bear in mind I am using Wikipedia to refresh my memory as the last time I read anything to do with the Categorical Imperative was 5 years ago (Christ, now I'm feeling old). Kant talks about Perfect Duty and Imperfect Duty, the former being logical statements that cannot be contradicted when we universalise them. For example, if we universalise theft then there would be no property, therefore it's illogical. Or if we universalise murder then there would be no lives, therefore it's illogical. Imperfect duty is more subjective, because you cannot logically exist in a constant state of performing that duty. Obviously you know what these mean, but I am summarising them in case anybody wants to pitch in.
That's obviously just the 'first formulation'.
Act only according to that maxim whereby you can at the same time will that it should become a universal law without contradiction.
The second formulation talks more about free will and that every action should not only have a principle but also be an end.
Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
And the third formulation. He talks about the 'kingdom of ends'. I'll quote the wikipedia article here because it offers a short and clear explanation:
Because a truly autonomous will would not be subjugated to any interest, it would only be subject to those laws it makes for itself — but it must also regard those laws as if they would be bound to others, or they would not be universalizable, and hence they would not be laws of conduct at all. Thus Kant presents the notion of the hypothetical Kingdom of Ends of which he suggests all people should consider themselves both means and ends.
Kant tries to cover all of his bases, but unfortunately situations arise where a maxim may be contradictory.
Murder is illogical because according to the first formulation and our perfect duty, to universalise murder would result in everybody dying, without life it is impossible to murder. On the second formulation, killing somebody denies their freedom unless they consent to it.
On the other hand:
Allowing another to murder is illogical because according to the first formulation and because it is not reasonable to 'universally stop murder', it would be an imperfect duty.
But on the second formulation. NOT allowing somebody to murder would be denying the murderer their freedom. Whilst, yes, the murderer is also denying their victim freedom, but stopping the murderer would then be using the murderer as a means to an end.
Say (a common hypothetical situation in ethical philosophy it seems) a terrorist about to blow up a school. It would be our imperfect duty to stop him, but we couldn't because stopping him would affect his freedom of choice. Say if it were permissible and if the only foreseeable way to stop him was to kill him then because murder is illogical and therefore wrong according to the categorical imperative then 'killing him' would not be a viable option. You're not getting his permission to kill him and killing him is a means to an end - ending his life to prevent him from killing lots of innocent children.
Kants criticises people, on their 'hypothetical imperative' (like Utilitarianism). However, a Utilitarian is able to reason that stopping a terrorist from blowing up a school full of children is a good idea. Because of how they judge morality based on what causes the great good for the greatest number and the least suffering for the smallest number - a large number of children's lives are saved and only 1 person dies in that scenario. I myself am not a utilitarian though, but I believe David Hume was a philosopher around the same period, so I think it's perhaps relevant.
Interestingly Kant and Hume were both empiricists.
Of course, I accept the possibility I have misunderstand Kant's philosophy. But I got an A on all my ethics exams I swear!
You do have the upper hand in having a better qualification in philosophy than I, so I am expecting all of my criticisms of Kant's ethics to be absolutely blown to smithereens.