Judges from the U.S. Court of Appeals for the District of Columbia Friday, in a lawsuit brought by four individuals who said the law violated their religious freedom, expressed skepticism about  the constitutionality of a key provision of the health care law.

At issue before the three-judge panel was the individual mandate,  a central provision of the Affordable Care Act, which requires individuals to buy health insurance by 2014 or pay a penalty.

Deputy Assistant Attorney General Beth  S. Brinkmann, the lawyer arguing for the Obama administration, said that Congress was well within its authority to pass the law, because health care costs across the country had spiraled out of control, and the Constitution authorized Congress to regulate interstate economic activity.

But Judge Laurence H. Silberman,  appointed to the federal bench by President  Reagan, questioned the limits of the government’s authority.

“What limiting principles do you articulate?” he asked Brinkmann. “What kind of mandate could the government come up with that would be unconstitutional?”

Silberman described a hypothetical situation in which the government could force an individual to buy a car.

Brinkmann responded that the mandate was not about a purchase but a type of financing. She called health care a unique market because it affects every individual sometime during their lives.

Judge Brett M. Kavanaugh, who was nominated to the bench by President George W. Bush, told Brinkmann he worried that if her position was upheld, down the road the government might  come back to court seeking to have other mandates in other markets upheld.

“Ten years from now,” he said, “I can see it coming.”

Judge Harry T. Edwards, appointed to the bench by President Jimmy Carter, suggested support of Brinkmann’s position  that health care should be differentiated from other markets.

Two of the four plaintiffs challenging the health care law on religious grounds said they didn’twant to be forced to buy health insurance because they believed that God would  take care of them, and they didn’t want to have to pay a penalty if they opted out.

In court, their lawyer, Edward L. White III of the American Center for Law and Justice, said that the Constitution did not have the authority to “force” his clients into a marketplace. One of the plaintiffs, Charles Edward Lee, said he believed in faith healing. White told the judges the mandate was “radical” and that while the Commerce Clause might allow Congress to regulate interstate economic activity, Congress did not have the right to regulate “inactivity” or the choice not to participate in the health care market.

Those following the legal challenges to the health care law were eager  to see how Friday’s arguments would go.

“Traditionally, the D.C. Circuit is thought of as the most powerful court in the country after the Supreme Court itself, and so even if it’s not the only court to speak to an important federal question, its views are often given even higher prominence than that of its sister circuits,” said Stephen I. Vladeck, a professor of law at American University Washington College of Law.

“In addition,” he said, “ in Judges Kavanaugh and Silberman, the three-judge panel includes two of the court’s leading conservative voices – and, many have assumed, two judges quite likely to look skeptically at the constitutionality of the ACA.”

Kavanaugh asked a long series of questions on whether a federal law, the Anti-Injunction Act, prevented challenges to the mandate from coming to court until the penalty  actually went into effect in 2014. The questions were significant because another  appeals court in Virginia dismissed a challenge to the law citing the Anti-Injunction Act.

“If the Supreme Court agrees that the Anti-Injunction Act prevents a pre-enforcement challenge,” said Kevin Walsh, a law professor at the University of Richmond School of law, ” then individuals will have to comply with the mandate or pay the penalty before they are able to get a court to decide whether it’s constitutional.”

The U.S. Court of Appeals for the D.C. Circuit is the fourth appellate court to hear arguments. One court has struck down the mandate, another has upheld it and a third dismissed the challenge on jurisdictional grounds.