The slogan that we're electing a Commander-in-Chief, not a Pastor-in-Chief, was one of the justifications some evangelicals gave to vote for Trump. Now, this post is not about the 2016 election. I'm discussing the slogan because it happened to crop up during the 2016 election, but the issue it raises is generally significant quite apart from the 2016 election.
i) What does the slogan mean? That's not entirely clear. In the nature of the case, slogans are apt to be intellectual shortcuts. There's not a lot of thought that stands behind the slogan, so it can be hard to pin down what it means.
However, I think the slogan trades on the stereotype that pious Christians are too otherworldly to make the tough calls that a president must make to protect us from our enemies. The slogan may have varied connotations, depending on who uses it, but that's the connotation I wish to focus on.
That's a very damaging stereotype. The notion that Christian ethics is too softhearted, too idealistic, to offer practical guidance in protecting the innocent. So at that point we resort to "pragmatism" and "consequentialism". We contract out the dirty work to unbelievers, who, because they don't suffer from Christian scruples, will do whatever is necessary to protect the innocent.
ii) In fairness, there are examples that play into that stereotype. The papacy, which used to be militaristic to a fault, has become pacifistic to a fault. You also have an influential interpretation of the Sermon on the Mount by Anabaptism. That's been influential, not in terms of how people act, but how people understand NT ethics. Many people agree with that interpretation, then conclude that it's hopelessly Pollyannaish, so they ignore it and do whatever it takes to defeat evil. By the same token, John Piper, who's often very useful, has become increasingly otherworldly.
iii) Conversely, many pious Christians are soldiers, or former soldiers. Off the top of my head, I can think of Joe Carter, David French, Ron Gleason, Rich Leino, Rick Phillips, Jimmy Li, Tony Perkins, and Robert L. Hubbard, Jr. They don't fit the stereotype of Christians who can't make the tough calls. And they illustrate the false dichotomy between a pastor and a combatant. At one time or another, you can be both. Indeed, some of them have been both.
iv) In addition, this debate is often connected to deontology. For instance:
The most familiar forms of deontology, and also the forms presenting the greatest contrast to consequentialism, hold that some choices cannot be justified by their effects—that no matter how morally good their consequences, some choices are morally forbidden. On such familiar deontological accounts of morality, agents cannot make certain wrongful choices even if by doing so the number of those exact kinds of wrongful choices will be minimized (because other agents will be prevented from engaging in similar wrongful choices). For such deontologists, what makes a choice right is its conformity with a moral norm. Such norms are to be simply obeyed by each moral agent; such norm-keepings are not to be maximized by each agent. In this sense, for such deontologists, the Right is said to have priority over the Good. If an act is not in accord with the Right, it may not be undertaken, no matter the Good that it might produce (including even a Good consisting of acts in accordance with the Right).
On the other hand, deontological theories have their own weak spots. The most glaring one is the seeming irrationality of our having duties or permissions to make the world morally worse…there are situations—unfortunately not all of them thought experiments—where compliance with deontological norms will bring about disastrous consequences. To take a stock example of much current discussion, suppose that unless A violates the deontological duty not to torture an innocent person (B), ten, or a thousand, or a million other innocent people will die because of a hidden nuclear device. If A is forbidden by deontological morality from torturing B, many would regard that as a reductio ad absurdum of deontology.
Deontologists have six possible ways of dealing with such “moral catastrophes” (although only two of these are very plausible). First, they can just bite the bullet and declare that sometimes doing what is morally right will have tragic results but that allowing such tragic results to occur is still the right thing to do. Complying with moral norms will surely be difficult on those occasions, but the moral norms apply nonetheless with full force, overriding all other considerations. We might call this the Kantian response, after Kant's famous hyperbole: “Better the whole people should perish,” than that injustice be done (Kant 1780, p. 100). One might also call this the absolutist conception of deontology, because such a view maintains that conformity to norms has absolute force and not merely great weight.
Some people, understandably enough, lose patience with ethics at that point. They feel it ties our hands. We become so scrupulous that we can't bring ourselves to stop atrocities. The extremes of strict morality and permissive morality meet.
v) However, one thing I'd like to point out is that deontology doesn't tell you where to draw the lines. Different deontologists draw different lines. For instance, some deontologists think lying violates an intrinsic duty never to lie, yet the mere theory of deontology doesn't spell out what our duties are. It's hard to say lying is intrinsically wrong unless you're a deontologist, but deontology, per se, doesn't entail a particular position on lying, suicide, abortion, contraception, euthanasia, capital punishment, or nuclear pacifism, &c. To affirm moral absolutes does not automatically select for what examples qualify as moral absolutes. The identification of specific candidates requires specific arguments, and not a general theory of duties. Once identified, the general theory will back that up. But a theory of duties doesn't ipso facto tell you where your duties lie.
My point is not to take a position on these issues, which I've discussed elsewhere, but to show that the impression some people have of deontology, is misleading.