Unlike his older colleague Justice Antonin Scalia, Justice Samuel Alito has never worn a beard. But to Alito, the U.S. Supreme Court's emerging leader on religious liberty exemptions, beards are ground zero.

Alito's most important opinion as an appellate judge turned on allowing Sunni Muslim police officers in Newark, N.J., to grow beards. And a beard is at the center of Holt vs. Hobbs, in which the U.S. Supreme Court heard oral argument this week on whether a Muslim Arkansas inmate may be forced to shave.

The facts of the Gregory Holt case are simplicity itself — but watch for the clue that explains why the Supreme Court took the case. Holt, who took the name Abdul Maalik Muhammad upon becoming a Muslim, asserts that as a good Muslim, he must allow his beard to grow. The state of Arkansas, where Holt is incarcerated, maintains a grooming policy for inmates — no facial hair except trimmed mustaches. The state's Department of Corrections claims the policy is necessary for security, because otherwise inmates might hide contraband, drugs or weapons in their beards, or rapidly change their appearance by shaving off their beards. Nevertheless, the state provides an exception: Any inmate with a diagnosed dermatological problem is allowed a beard, provided it is trimmed to a quarter-inch length.

Because Holt is a state prisoner, the Religious Freedom Restoration Act, which the Supreme Court used in June to grant Hobby Lobby an exception from the Affordable Care Act, doesn't apply. So Holt brought suit under the Religious Land Use and Institutionalized Persons Act (RLUIPA). This law applies to all prisoners in facilities that receive federal support, which includes Arkansas prisons. It says that the government may not substantially burden an inmate's religious exercise unless the burden "(1) is in furtherance of a compelling governmental interest; and (2) is the least restrictive means of furthering that compelling governmental interest."

So far, so good: Arkansas has to show that it has a compelling interest in banning beards and that the ban is the least restrictive means of accomplishing that interest. If this burden sounds hard to meet, that's because it supposed to be. In essence, the statute states the legal doctrine known as "strict scrutiny," which is applied to government acts that burden fundamental constitutional rights.

In practice, however, prisoners very rarely win RLUIPA cases. Courts tend to be intolerant of religious exemptions demanded by prisoners who, they imagine, have nothing better to do than convert to a new religion and dream up exceptions from prison rules. Courts avoid granting RLUIPA exceptions by using one of the cleverest techniques in the judge's toolbox: They systematically defer to prison officials' determination of what counts as a compelling interest in prison security. Sounds reasonable enough. After all, who better than prison officials would know how to run a prison safely? Yet a norm of deference, which is not required or even mentioned by RLUIPA, has the effect of blocking almost every religious exemption that could be imagined. A cynic would even say that this deference guts RLUIPA — and that judges like it for that reason.

Holt's case followed the norm. A federal district court and then the U.S. Court of Appeals for the 8th Circuit concluded that the state had met its burden. Under 8th Circuit precedent, the court deferred to the prison system in the absence of substantial evidence that the prison security concerns were exaggerated. The fact that other prison systems allow beards, the court said, wasn't enough to override this deference. Holt lost.

The Supreme Court would not have agreed to hear this garden variety case unless at least four of the justices wanted to reverse. Yet the justices would be loath to reverse the lower courts' use of deference, which helps keep the dockets clear. So why did the court take the case? There's a tiny clue in the facts of Holt's case that strongly suggests why.

Did you catch it?

The clue is the exception that the Arkansas Department of Corrections makes for people with skin conditions, who are allowed to grow a quarter-inch beard. In the Hobby Lobby case, the Supreme Court — in an opinion by Alito — said the Obama administration couldn't claim that it had adopted the least restrictive means necessary to extend contraceptive insurance coverage, because it had made an exception for nonprofit religious associations such as groups of nuns who were exempted from for that coverage for their employees.

Following this logic, the fact that the Department of Corrections makes an exception for men who can't shave must be evidence that it hasn't adopted the "least restrictive" means of maintaining safety by banning beards. If a few people can have short beards, why can't all?

Alito actually dreamed up this logic in a 1999 case, Fraternal Order of Police vs. City of Newark. The city banned not inmates but police officers from wearing beards. It made an exception, however, for officers suffering from folliculitis.

Supreme Court precedent ordinarily denies constitutional exemptions when there is a neutral, generally applicable law in place. (Scalia set that precedent in Employment Division vs. Smith, 1990.) In a subversively brilliant reinterpretation of the Smith precedent, then-Judge Alito said that the exemption must be granted because the city had created a system of individual exemptions. Because it allowed medical beards, the city had to allow religious ones.

Today, the beard is coming home to roost. Expect the court to let Holt grow his — and expect Alito to write the opinion.