Cardinal Donald W. Wuerl,archbishop of Washington, greets workers before a blessing of the Trinity Dome at the Basilica of the National Shrine of the Immaculate Conception. (Jabin Botsford/The Washington Post)

It was a holy moment at high altitude.

With his miter nearly brushing the 159-foot ceiling, Cardinal Donald W. Wuerl stood atop 620,000 pounds of steel and scaffolding Friday morning to bless the dome of the Basilica of the National Shrine of the Immaculate Conception.

“If we were to have a hymn to open this entire celebration, I think it would be ‘Nearer, My God, to Thee,’ ” joked Wuerl, the archbishop of Washington. “This is about as close as we are going to get physically to that experience.”

The unusual ceremony marked not only the completion of the scaffolding on which Wuerl stood but also the beginning of the end of 100 years of construction on the country’s largest Catholic church, where Pope Francis canonized a Spanish missionary last year.

The plain gray plaster walls of the dome, which the cardinal splashed with holy water, are the last item on a century-old checklist.

A mosaic that was blessed by Pope Francis is propped up against the scaffolding inside the Basilica of the National Shrine of the Immaculate Conception. (Jabin Botsford/The Washington Post)

Since the church in Northeast Washington opened, unfinished, in 1959, the unadorned Trinity Dome has seemed out of place in the increasingly ornate building. (Two smaller domes were completed in 2005 and 2006.) With the scaffolding now in place, workers will spend January through August installing a massive mosaic. Church officials hope to have it on display by Christmas next year.

The Italian-made mosaic will consist of more than 14 million pieces — in more than 1,000 colors — of Venetian glass and will feature images of Jesus, Mary and the 13 American saints. It will span 18,300 square feet of the dome surface.

“It will be a wonder to behold,” Wuerl said during the ceremony.

For many in attendance, however, the real wonder was already underneath their feet.

Jon Tung watched the proceedings with a mixture of pride and relief. As the structural engineer, he had nervously watched as the project slowly took shape. Eight huge steel beams were erected inside the renowned sanctuary. Then, in a single day, a 180-ton platform was hoisted 60 feet above the nave.

“This is the most complex project I’ve ever worked on,” Tung, 45, said. “There were a lot of sleepless nights.”

Descending a staircase, he gestured toward a handful of visitors wandering the pews six stories below, oblivious to the construction above them. “You can imagine the worst-case scenario,” Tung said.

A construction worker stands amid the sprawling scaffolding after the blessing of the construction on the Trinity Dome. (Jabin Botsford/The Washington Post)

Eight floors of scaffolding were then built on top of the platform, all the way to within eight inches of the church’s ceiling.

“It’s an in­cred­ibly, intensively engineered structure,” said Roger A. Jetton, president of Scaffolding Solutions. His company brought in the 20,000 interlocking pieces from Glasgow. It took 30 workers more than five weeks to build the maze of metal. “I’m very proud,” he said.

So was Rafael Aleman, one of the workers who assembled the scaffolding. Before the ceremony, the cardinal stopped by to shake Aleman’s calloused hand.

“Thank you for what you’re doing,” Wuerl said.

“You’re welcome,” answered Aleman, a 39-year-old Catholic who was born in El Salvador and lives in Richmond. This job was unlike the power plants and factories he usually works on, he said. “It’s the house of God.”

Aleman and his co-workers had sent videos of themselves working in the shrine to their families, who weren’t allowed to visit. “We’ll bring our families to see it when it’s done,” he said.

At the beginning of the ceremony, Monsignor Walter R. Rossi, the rector of the basilica, prayed in English and Spanish for the safety of the workers on the $20 million project. With its many pieces and colors, he said, the mosaic would reflect the “most diverse congregation in the country.”

Wuerl said he had first come to Catholic University, the shrine’s home, 66 years ago as a college student. When the dome is finished, he said, visitors of all faiths will be “enveloped in a beauty they might otherwise never experience.”

Near the end of the service, the cardinal made an unusual request of the several dozen construction workers, priests, nuns and reporters on hand.

“Touch the dome before you go down, because you’ll never get a chance to touch it again,” he said.

Joseph Chee, a member of the blue-robed choir that sang at the event, took Wuerl up on the offer — and then some.

With a black pen, the 23-year-old scrawled his initials on the gypsum plaster, which will be stripped over the next month so that concrete can be added ahead of the mosaic. He then marveled at the scaffolding beneath him. “It’s like they are building a spaceship up here,” he said.

Chee, who graduated from Catholic University in 2015 and lives nearby, said the shrine felt like a second home to him. For a long time, he wasn’t sure if the dome would ever be completed. Now he had faith.

“It will be nice to see my home when it is done,” he said.