This year marks that dedication's 1,700th anniversary.

The dedication's feast day is a required liturgical celebration on the Roman liturgical calendar, which sets it apart from other optional feasts commemorating Rome's three other major basilicas -- the Dedication of the Basilica of St. Mary Major Aug. 5, and the Dedication of the Basilicas of Sts. Peter and Paul, Apostles (St. Peter’s Basilica and the Basilica of St. Paul Outside the Walls) Nov. 18.

That's for the same reason St. John Lateran's facade bears the Latin inscription of "Mother and Head of all the Churches in the City and the World," explained Father John Wauck, who teaches literature and communication of the faith at the Pontifical University of the Holy Cross in Rome. As the cathedral of the Diocese of Rome, it -- not St. Peter's Basilica -- is the church of the bishop of Rome, the pope. And as the mother church of Rome, it is united to all other churches in the world.

In each diocese, the anniversary of its cathedral's dedication is observed as a feast day, so with St. John Lateran, that observance is extended to the universal church, Father Wauck said.

The importance the church places in its sacred buildings connects to Jesus' ministry, he said. "What he came to do was a construction project," Father Wauck said. "He tells Peter, 'On you, on this rock, I'm going to build my church. … He doesn't say, 'I'm going to write a book.' He doesn't leave any books. He leaves followers to form what he calls a church, and he says he's going to build it. … The physical church is like the incarnation of the spiritual thing that is a church."

Dedicated to Christ the Savior in 324, St. John Lateran was later placed under the additional patronage of St. John the Baptist and St. John the Evangelist, with the formal name of the Papal Archbasilica of the Most Holy Savior and Sts. John the Baptist and John the Evangelist at the Lateran. "Lateran" refers to its location on a hill that was formerly the Roman estate of the Laterani family. It is also the only Catholic church styled as an "archbasilica."St. John Lateran's 1,700th anniversary is more than a milestone for a particular church, said art historian Elizabeth Lev. As the first legally-built public church in the world, it marks the legalization of Christianity and the advent of public Christian worship, she said.

"It establishes the moment that Christianity can make its presence publicly felt in the city," said Lev, a college instructor and sought-after Roman tour guide.

The Roman Emperor Constantine the Great legalized Christianity with the Edict of Milan in 313 after he claimed a vision from God on the eve of the pivotal Battle of the Milvian Bridge in 312. That vision inspired him to adhere symbols for Christ on his army's banner, and his army was victorious. He gave property to the pope for the construction of the archbasilica.

Much of what is known about the building of St. John Lateran, Lev said, comes from the historian Eusebius, bishop of Caesarea in Palestine and a contemporary of Constantine, who grew up under Emperor Diocletian's severe persecution of Christians. With the legalization of Christianity, Lev said, he wrote that the Christians of his day had lived to see something for which the martyrs "craved to see and did not."

"This is a very important thing for us to remember: That church is built on the witness of 250 years of people willing to die for Christ," Lev said. "It's a reminder that we have this incredible privilege of being able to go out and proclaim our faith."

Among its prominent artwork is a series of larger-than-life statues of the Twelve Apostles added ahead of the basilica's reconstruction for the 1650 Jubilee Year, some with symbols of their martyrdom.

"There's a wonderful St. Bartholomew holding his own skin, which is draped over the front; there's Matthew rejecting a bag of coins," Lev said. "But most beautifully, halfway down the nave, there is the statue of St. Thomas," with his finger protruding from the niche and pointing to the altar.

"I think that's a very strategic placement of this sculpture, because It reminds us that in this journey … there's always a moment of doubt when we wonder if we're on the right track, we're during the right thing," she said. At St. John Lateran, "there is Thomas, with the most famous finger in the Gospels, pointing the rest of the way."

The fragment of an earlier fresco by Giotto shows Pope Boniface VIII standing in St. John Lateran declaring the first Jubilee Year in 1300 -- a visual connection to the upcoming Jubilee Year in 2025 declared by Pope Francis.

St. John Lateran includes one of the oldest extant baptisteries in Christendom, ornate spiraling columns in its medieval cloister, and a glittering mosaic of Christ and saints in its apse.

The basilica is also home to important relics such as the skulls of Sts. Peter and Paul, part of the table of the Last Supper, and, outside in its own chapel, the Scala Sancta, or the holy steps said to be the ones Jesus climbed to Pontius Pilate in Rome, brought to Rome by Constantine's devout Christian mother St. Helena.

The central, papal altar contains the remnants of an old wooden table that is said to have served as St. Peter's altar when he was bishop of Rome, which, Lev said, "is a reminder that when Peter was here running the church, as it were, he was always looking over his shoulder."

"This was an illegal religion that was being hunted down by Romans ... so Peter really isn't really established anywhere, he's just going from place to place," she said. "When they built this church that wooden altar could come to rest inside a stone altar, because it's not going anywhere."

While construction on St. Peter's fourth-century basilica -- known as Old St. Peter's Basilica -- began just a short time after that of St. John Lateran, that building was demolished in the 16th century, and St. Peter's Basilica was completely rebuilt, resulting in a coherent Renaissance style. By contrast, St. John Lateran's evolution over the centuries is evident with architectural and art styles of every period. However, after countless renovations, very little of the original building is visible, Lev said.

Duncan Stroik, a practicing architect who teaches at the University of Notre Dame School of Architecture, said that the range of art in St. John Lateran is an exemplar of artistic continuity through its stylistic diversity.

"It's the first church, and it's been changed and added to, and you see all of these different layers," he said. "You see a cosmatesque floor, you see the Renaissance ceiling, you see the beautiful transept done in the Holy Year of 1600 that's focused on the Eucharist, you have the apse with the throne of the bishop" in a medieval revival style from the 19th century, "and a baroque nave."

Despite its art, history and honorifics, however, St. John Lateran is far less visited than St. Peter's Basilica in Vatican City, which Pope Francis recently said draws 40,000 visitors each day. By contrast, a person can easily enter St. John Lateran without lines and "you have the place to yourself," Stroik said.

If in Rome the architectural glory of St. Peter's Basilica is the Catholic Church's "game face," making the Catholic Church's long history "look easy," then St. John Lateran reveals the flip side, Lev said.

"St. John Lateran reminds us that 2,000 years is not that easy," Lev said, noting that the church has endured earthquakes, fires and sacking by invaders. "It really had to be reconstructed over and over and over again," she said, adding that that history makes it a symbol of the church itself.