Alan Keyes says, "If citizenship is not a birthright then it must be a grant of the government. And if it is a grant of the government, it could curtail that grant in all the ways that fascists and totalitarians always want to."
In other words, the rights vis-a-vis the state we call citizenship, are prior to the legal acts that formalize them.
Joshua Micah Marshall thinks that's "dramatically crazier than any of the opinions on offer," since Keyes attributes the priority of citizenship, in part, to God.
But as a historical matter, Keyes is certainly right. The founding documents of political liberalism -- the Declaration of the Rights of Man and Citizen, the Declaration of Independence -- explicitly state that the rights of the citizen are prior to their recognition by governments. If a government fails to recognize them, it's that government's legitimacy that is diminished, not the rights of the citizen.
In the specific 14th Amendment context, the point is that the right of the freedmen to citizenship wasn't created by the 14th Amendment, but already existed by virtue of their living in this country and being subject to its laws. Did Congress have the power or the authority to deny them citizenship? Seems to me the Civil War answered that question clearly in the negative. The law binds most of the time, but ultimately it derives its authority from a set of norms that are prior to it.
This is certainly how the founders of liberal political orders, here and elsewhere, understood the relationship between the rights of the citizen and the law. That's why they were ready to overthrow existing governments by force. Of course today it's the Constitution and the law that regulate citizenship. But it's important to remember that the fact that we -- or almost anyone else -- are citizens at all is not the result of legal or constitutional acts.
EDIT: It's funny that reference to the founding documents of political liberalism is these days almost a monopoly of conservatives. Of course it's not so strange, since conservatism is backward-looking by nature, while progressives naturally believe in progress. But the DNA of liberalism hasn't changed that much, and Jefferson, Madison, and Hamilton, Lafayette and Saint-Just, and other Enlightenment political figures expressed it pretty robustly.
Unlike their forebears, modern liberals tend to insist on the absolute autonomy of the law in general, and the Constitution in particular. They're unwilling, for obvious reasons, to accept a political order grounded on divine revelation, but they don't have any alternative ground to put it on, so it ends up floating in the air. (Carl Schmitt is very good on this.) There's what's useful, and there's what's legal, under the law as it exists; but there's no category of political legitimacy behind the law. Given the remarkable political stability of the United States since the Civil War, and just as important, as Herbert Croly emphasized, the continuously rising standard of living here, we've mostly gotten along fine without one. But one suspects that it wouldn't take that much political strain for "government by lawyers" (Croly's phrase) to experience its Wile E. Coyote moment, when it turns out that the authority of the law wasn't underpinned by anything but a lack of good reasons to question it. Not unlike, perhaps, what happened in the financial crisis of 2008, when it turned out that not only did the traditional tools of monetary policy not work, they'd stopped working some time before.