Immanuel Kant does not always receive the fairest treatment among self-styled conservative theologians.
I have read works in which his whole philosophy is caricatured and dismissed in a single paragraph—hardly charitable treatment of one of the most brilliant minds of the modern era. The motivation tends to be that Kant's philosophy creates problems for some traditional Christian convictions, such as the possibility of recognizing supernatural revelation or obtaining knowledge of God or the importance of historicity to the Christian religion. Rather than engage his philosophy justly, giving it credit where due and honestly struggling with such problems, many take the easy road of rejecting it off hand.
Doing so, however, runs the risk of overlooking areas of concordance in which conservative theologians and Kantian philosophers may have something to offer one another. Oddly, the second century Christian saint and apologist Athenagoras defended the doctrine of the resurrection on—we may anachronistically say—Kantian grounds that may serve to demonstrate the value of such dialogue between ancient theology and modern philosophy, Kant in particular.
To begin, I must briefly summarize Kant's categorical imperative. While I would caution that prudence sometimes calls for more attention to situational particulars than Kant—or the common caricature of Kant—seems to want, the principle itself, in at least one of its formulations, is a good one and one that has been taken seriously as a foundation for personalist ethics.
In his Groundwork for the Metaphysics of Morals, Kant writes,
Beings whose existence depends not on our will but on nature’s, have nevertheless, if they are irrational beings, only a relative value as means, and are therefore called things; rational beings, on the contrary, are called persons, because their very nature points them out as ends in themselves, that is as something which must not be used merely as means, and so far therefore restricts freedom of action (and is an object of respect). These, therefore, are not merely subjective ends whose existence has a worth for us as an effect of our action, but objective ends, that is, things whose existence is an end in itself; an end moreover for which no other can be substituted, which they should subserve merely as means, for otherwise nothing whatever would possess absolute worth; but if all worth were conditioned and therefore contingent, then there would be no supreme practical principle of reason whatever. If then there is a supreme practical principle or, in respect of the human will, a categorical imperative, it must be one which, being drawn from the conception of that which is necessarily an end for everyone because it is an end in itself, constitutes an objective principle of will, and can therefore serve as a universal practical law.
Kant here makes a distinction between irrational and rational beings. Irrational beings existing in nature are things and able to be used by us for our ends. Rational beings, on the other hand, are persons, "whose existence is an end in itself." Each rational being regards itself in such a way but cannot morally do so unless this subjective principle is raised to be objective, i.e. applying it to all others as well. Thus he summarizes: "So act as to treat humanity, whether in thine own person or in that of any other, in every case as an end withal, never as means only."
For Kant this is a supreme, universal, practical law, a purely rational foundation for morality. As a simplistic example, we may apply this law variously as follows: If each person exists as an end in themselves, one person cannot enslave another as a means to economic end; or murder another as a means to satisfy one's passion of anger or jealousy; or steal from another, reaping the fruit of another's labor or otherwise using their possessions as if they were one's own; or lie to another deceiving that person into believing or doing one's own will when they otherwise would not; and so on. It is one way of saying that human persons qua persons have inalienable rights and thus are "object[s] of respect," whose existence is irreplaceable, thus restricting our "freedom of action" towards them. All immorality reduces persons to be used as unwilling means for others. All moral behavior treats others with the utmost dignity and "absolute worth."
Rewind now to the second century and consider St. Athenagoras. In his work On the Resurrection of the Dead, defending the resurrection against charges that it was irrational, he writes,
God can neither have made man in vain, for He is wise, and no work of wisdom is in vain; nor for His own use, for He is in want of nothing.... Neither, again, did He make man for the sake of any of the other works which He has made. For nothing that is endowed with reason and judgment has been created, or is created, for the use of another ... but for the sake of the life and continuance of the being itself so created... [I]rrational beings are by nature in a state of subjection, and perform those services for men for which each of them was intended....
Here we see a similar distinction to the one Kant draws. Among beings in nature, i.e. created by God, there are irrational and rational beings. An irrational being exists for the use of rational beings, but a rational being exists "for the sake of the life and continuance of the being itself so created."
St. Athenagoras continues,
Therefore ... it is quite clear that although, according to the first and more general view of the subject, God made man for Himself, and in pursuance of the goodness and wisdom which are conspicuous throughout the creation, yet, according to the view which more nearly touches the beings created, He made him for the sake of the life of those created, which is not kindled for a little while and then extinguished....
Considered according to our moral and spiritual ends, we are made for God, but considered from God's perspective, he is need of nothing. St. Augustine is right to pray, "Thou madest us for Thyself." The longing of our hearts cannot be satisfied apart from communion with God. But on the other hand, considered on the level of our mere existence, God made us to exist for ourselves and not to be used by others.
St. Athenagoras goes on:
[T]hat which was created for the very purpose of existing and living a life naturally suited to it, since the cause itself is bound up with its nature, and is recognised only in connection with existence itself, can never admit of any cause which shall utterly annihilate its existence.
If God created us to exist as ends in ourselves, we cannot allow the idea that our annihilation could be tolerated—we are not created to be used and disposed of by others. Otherwise God would have poorly made humanity for the purpose of "existing and living a life naturally suited to it." Such an idea hardly befits the wisdom of God.
Coming to the resurrection, then, St. Athenagoras proceeds,
But since this cause is seen to lie in perpetual existence, the being so created must be preserved for ever, doing and experiencing what is suitable to its nature ... so that the soul may exist and remain without change in the nature in which it was made, and discharge its appropriate functions (such as presiding over the impulses of the body ...), and the body be moved according to its nature towards its appropriate objects, and undergo the changes allotted to it, and, among the rest (relating to age, or appearance, or size), the resurrection. For the resurrection is a species of change, and the last of all, and a change for the better of what still remains in existence at that time.
If we believe that human persons exist as ends in themselves and that God is wise and did not and could not make us in vain, we must also believe that the body will have its existence restored to it someday through that ultimate "change for the better": the resurrection. The body is an essential part of human persons, and one of the "appropriate functions" of the rational soul is "presiding over the impulses of the body." Thus, for each human person, who is body and soul and whose existence is an end in him/herself, to continue "in perpetual existence" and carry out the functions "suitable to its nature," the body must rise again and be reunited with the soul. Or, to put it better, "this corruptible must put on incorruption" (1 Corinthians 15:53).
Thus, based on the same principle and foundation as Immanuel Kant's categorical imperative, St. Athenagoras defended the doctrine of the resurrection. It may not have been one of the chief concerns of Kant's own philosophy, but if we find it compelling in defense of dogma, we ought also to consider seriously the many other ways in which Kant put it to use in developing his moral philosophy.