A reflection of changing beliefs.

This one is for the ages. Justice Anthony M. Kennedy's opinion for the U.S. Supreme Court announcing a right to gay marriage in Obergefell v. Hodges will take its place alongside Brown v. Board of Education and Loving v. Virginia in the pantheon of great liberal opinions.

One notable contrast with those earlier landmarks in the history of equality is that both of those were decided unanimously. Friday's gay-rights opinion went 5-4, with each of the court's dissenting justices writing an opinion of his own. Eventually, legal equality for gay people will seem just as automatic and natural as legal equality for blacks. But history will recall that when decided, Obergefell didn't reflect national consensus, much less the consensus of the court itself.

Kennedy's opinion offered two different yet interrelated constitutional rationales, one focused on the institution of marriage, the other on the equality of gay people.

First, he made the case that marriage was a fundamental liberty right under the due-process clause of the Constitution, which says no one may be deprived of life, liberty, or property without due process of law.

Applying what's known as "substantive" due-process analysis, Kennedy held that the government may not infringe the liberty to marry absent a compelling interest and along narrowly tailored lines to achieve that interest. Because no such interest exists, gay people as well as straight people must have the right to marry. This same approach was used by the court in the Loving case, which struck down laws barring interracial marriage. It was symbolically important for Kennedy to connect same-sex marriage to marriage between the races.

Kennedy's favorite concept of dignity figured large in the finding that marriage was a fundamental right: "The lifelong union of a man and a woman always has promised nobility and dignity to all persons, without regard to their station in life." The reference to dignity connected the decision to Kennedy's earlier gay-rights decisions, which featured the concept centrally. It is now an important part of our constitutional law - no matter that it doesn't appear in the Constitution.

Another crucial feature of the opinion was Kennedy's recognition that marriage has evolved over time. This acknowledgment counteracted the emphasis on tradition in the dissents. It also resonated with the doctrine of due process, which looks to evolving tradition to identify the content of protected liberty.

When it came to equality, Kennedy avoided announcing that laws burdening gay people would be subject to especially strict scrutiny, like laws burdening racial minorities, or even what's called intermediate scrutiny, like laws differentially burdening the sexes. Instead, he spoke of the "synergy" between due process and equality. In legal terms, this almost certainly meant that once a fundamental right was invoked, any distinction between people for any reason required strict scrutiny - a longtime doctrinal norm.

But Kennedy didn't quite say so, probably because he wanted to preserve the legacy of earlier gay-rights opinions in which he never said that he was engaged in close scrutiny, but said rather that the laws were discriminatory on their face.

Chief Justice John G. Roberts Jr., an advocate of judicial restraint, was in a position to criticize Kennedy's opinion for judicial activism - which it unquestionably is. Roberts cited one of the Supreme Court's most derided opinions, Lochner v. New York, in which a property-protecting libertarian majority struck down a working-hours-limitation law for violating what it called the "liberty of contract."

Among lawyers, the Lochner case stands for unbridled activism and the imposition of an ideology not found in the Constitution.

Analytically, Roberts is right that Friday's decision reflects changing beliefs about what counts as liberty and equality. Where he is wrong is in thinking that the public will generally reverse itself on the topic of gay rights. When it comes to equality and liberty, the modern trend is to extend rights, not contract them. Roberts' dissent is truly unfortunate, therefore, for his legacy: It reflects a commitment to judicial restraint, but in the long run, it will be seen as having weighed in on the wrong side of history.

That left it to Justice Samuel A. Alito Jr. to try to defend the traditional definition of marriage - and to defend its defenders. The decision, he wrote, "will be used to vilify Americans who are unwilling to assent to the new orthodoxy." Here's hoping he's wrong about that.

The truth is that religious liberty remains a central American value. The gay-marriage decision shouldn't touch that First Amendment tradition, either legally or symbolically. The case is a victory for liberty and equality - and that's the sort of orthodoxy everyone should be able to embrace.