The visit of Pope Francis to Philadelphia is a cause for celebration. The pontiff has won the hearts of people around the world, and his time here will be a blessing for many reasons.

This is also an opportunity to avoid the constitutional violations made when Pope John Paul II visited in 1979. Justice William Brennan, an observant Catholic who went to church daily, was a staunch defender of the constitutional prohibition on state aid to religion because the hand that giveth may also restrict — even destroy.

This was the lesson the city failed to learn in 1979, when Mayor Frank Rizzo announced, "I will build a church for my pope." I was then general counsel for the American Civil Liberties Union in Philadelphia. I responded that the ACLU would sue to prevent such a flagrant, brazen violation of the First Amendment. Rizzo refused to yield.

The city proceeded to build a platform in Logan Circle across from the Cathedral of SS. Peter and Paul and to erect a giant cross, all for the purpose of the pope's conduct of a Catholic Mass. It was all at the expense of the city.

I made the decision to sue to stop the city and I selected Henry Sawyer III to conduct the litigation. Sawyer was the most distinguished litigator in church-state matters of his time. Many volunteers of several religions asked to be plaintiffs and a Catholic was chosen to emphasize that this was not a sectarian cause but a matter of the U.S. Constitution. Thus, the case of Gilfillan v. City of Philadelphia was filed in federal court.

The ACLU did not sue the pope or the church, only the city to stop building the platform and supplying the cross. Other municipal expenses for the pope's visit were not challenged.

The case was assigned to Judge Raymond Broderick, a Catholic who was involved with the pope's visit. The question arose whether to ask Broderick to disqualify himself but Sawyer, in his finest hour, rose above the fray and proclaimed that the Constitution provides that it "shall be the supreme law of the land," and Broderick could be relied upon to obey its command. That ended the discussion.

Rizzo asserted that the ACLU wanted to stop the pope from saying Mass, which was not true, and in other cities the organization had gone to court to protect the pope's freedom of religion. We agreed that the Mass should proceed and, if the city was found to violate the Constitution, the church should reimburse the city for the expense of the platform and cross.

The court ruled that the city violated the Constitution, and that was affirmed on appeal.

The city and church then asserted that the cross should remain in place in Logan Circle as an historic marker that Pope John Paul II had conducted a Mass there. When the ACLU threatened to return to court to prevent another church-state violation, the city backed down. The church moved the cross to its St. Charles Borromeo Seminary on City Avenue. That is where it belongs, in perpetual vigil to the First Amendment protection of freedom of religion and the separation of church and state.

There were fascinating aspects to the case, both in and out of court, and much has been written about it. There never was any doubt as to the constitutional righteousness of the ACLU's lawsuit to affirm the principle that government must obey the Constitution even when the pope is involved.

That did not restrain the flood of accusations that the ACLU was against Catholics and a right-wing Philadelphia newspaper columnist charged that ACLU stood for "Anti-Catholic Litigation Unit." The Pittsburgh Post-Gazette characterized those defending the First Amendment as "Jewish ACLU-types." The torrent of hate mail in the press was unprecedented but that received by ACLU was terrifying.

One letter in particular I cannot forget. Our executive director, Hilda Silverman, was threatened with murder. "As for Burton Caine," the epistle continued, "these Protestants are all alike."

As to the amount the city paid for the pope's Mass, the Wall Street Journal commented that only a fraction of the cost was paid by the church because the city excluded the value of labor and property furnished on the ground that that did not cost anything. However, the ACLU did not contest the cost in order to focus on the First Amendment principle of separation of church and state.

It is well to recall this history even as the city promises not to repeat it. Mayor Nutter says that no city funds will finance prayers of Pope Francis in Philadelphia. That will foreshadow a positive history of obeying the Constitution. It recalls a story that Justice William Brennan was fond of telling:

In a small town, before there were electric street lamps, a lamp lighter would light each lamp by hand. He placed his ladder against each lamp, climbed up, lit it, and moved on to the next one. And so he would continue until he was out of sight, but all could see the path taken.

After a wrong turn, the City of Philadelphia vows to follow the constitutional path in the city where that charter of liberty was born.

Burton Caine is a professor of law at Temple University School of Law and past president ACLU-Philadelphia.