The following script is from "God's Architect" which aired on March 10, 2013. Lara Logan is the correspondent. Max McClellan, producer.
During his eight years as pope, Benedict XVI carried out thousands of official duties, but only once did he travel outside Rome to bestow the Vatican's highest honor on a church, transforming it into a basilica -- a sacred place forever.
Tonight, we're going to take you to that extraordinary church. It's called the Sagrada Família and, if you've ever been to Barcelona, Spain, you couldn't have missed it. It may be one of the most spectacular buildings ever constructed by man, the vision of genius Spanish architect, Antoni Gaudí, known as God's architect, who died almost a century ago. It's been under construction for 130 years and it's still not finished.
Why would a church take so long to build? Because Gaudí's design was as complicated as it was advanced. Today, the Sagrada Família has become the longest running architectural project on earth.
When Pope Benedict came to the Sagrada Família two years ago, it was the first time mass had ever been held here. In an ancient tradition as old as the Catholic Church, he consecrated the Sagrada Família as a basilica.
Not since 1883, when it was envisioned by Antoni Gaudí, had it been seen in all its glory.
Eight hundred voices filled the air, one of the largest choirs in the world and close to 7,000 people gathered, celebrating a moment that had taken 128 years to arrive.
While the inside is mostly finished, outside there's still much to be done. You can see the spires and construction cranes for miles.
Watch as this picture moves in from above -- those tiny figures below are people dwarfed by the massive facade, rising from the main entrance of the church.
Antoni Gaudí was profoundly devout and this was his way to make amends to God for the sins of the modern world.
Gijs van Hensbergen: I mean, he wanted to write the history of the whole of the Catholic faith in one building. I mean, how crazy and how extraordinary and how ambitious and how, in a sense, megalomaniac that idea is.
Gijs van Hensbergen immersed himself in Antoni Gaudí's life for 10 years and wrote what's considered the definitive biography. He took us to see the Nativity Facade, the only part built while Gaudí was alive.
Gijs van Hensbergen: It's the Bible written in stone.
Lara Logan: So every single little thing that you look at there, every detail symbolizes something real?
Gijs van Hensbergen: Yea, and that was the idea, that we together would spend days here, me teaching you if I was a priest, what the story was and what the symbolism was. And once you get inside is a wonderful, kind of spiritual boost.
The ceiling is a striking display of Gaudí's engineering genius. He wanted the interior of his church to have the feel of a forest because that's where he believed man could feel closest to God. And when you look upwards, you can see Gaudí's columns branching out like trees.
Gijs van Hensbergen: Trees are actually buildings, he said. It knows where to throw out a branch. And if you look at the Sagrada Família today that's exactly what happens with those bizarre, eccentric-- they look bizarre and eccentric but the engineering beneath it is absolutely exceptional.
Van Hensbergen pointed out that, as you move towards the altar, the columns are made from stronger and stronger stone. Gaudí chose red porphyry from Iran, for the ones that bear the heaviest load, because it's among the strongest in the world.
Lara Logan: If you had to define, sort of, the one thing, that distinguished Gaudí as an architect, what would it be?