Lodged in the heart of Rome, Vatican City is the spiritual capital of the Roman Catholic world.

Here’s a case where crossing a street is crossing a border. I just left Italy. Vatican City may be the world’s smallest independent country, with just a thousand inhabitants, but it’s the spiritual capital of hundreds of millions of Roman Catholics.

The Vatican is built upon the memory and grave of the first pope, St. Peter. Its centerpiece: St. Peter’s Basilica.

Even though Vatican City occupies less than a square mile — this country has its own radio station, newspaper, post office, and a cute little train station. Along with the grandest church on earth, it has a massive museum. Vatican City — ruled both politically and religiously by the pope — is embedded in the city of Rome. It’s surrounded by a mighty medieval wall that evokes a less-than-peaceful history.

The Vatican “military” is made up of the Swiss Guard. In 1506, the pope imported mercenaries from Switzerland, who were known for their loyalty and courage. Today, about 100 Swiss soldiers — clad in their flamboyant Renaissance-style uniforms — still protect the pope, keep the crush of visitors in order, and patiently answer tourists’ questions.

Piazza San Pietro sits on the site of an ancient Roman racetrack. Imagine chariots making their hairpin turns around that obelisk.

For added entertainment during the races, Christians were executed right here. In about AD 65, the Apostle Peter was crucified within sight of this obelisk. Peter’s friends buried him in a nearby graveyard on what pagan Romans called the Vatican Hill.

For 250 years Christians worshipped quietly at his tomb. Then, in AD 313, Emperor Constantine legalized Christianity and changed the course of history. A basilica was built here, and this became the head of the Roman Catholic Church.

Twelve hundred years later, the original St. Peter’s was replaced by this most glorious church in Christendom. Upon entering, your first impression is: It’s huge…600 feet long, bathed in sunbeams. It can accommodate thousands of worshippers. As a tour guide, I’ve lost entire groups in here.

Michelangelo’s Pietà is adored by pilgrims and tourists alike. Here the 24-year-old Michelangelo makes the theological message very clear: Jesus — once alive but now dead — gave his life for our salvation. The contrast provided by Mary’s monumental robe makes his body — even though carved in stone — feel soft and believable.

Shining in the apse, at the far east end, featuring the Chair of St. Peter, is another masterpiece by Bernini. With sunlight illuminating its alabaster window — as if powered by the Holy Spirit — it lifts up the legendary throne of the first pope with a starburst of Baroque praise.

Directly above the altar, rising above the tomb of St. Peter, stands Bernini’s bronze canopy, and above that, Michelangelo’s dome — taller than a football field on end. The inscription declares, in Latin: Tu es Petrus…“You are Peter and upon this rock I will build my church.” This is the scriptural basis for the primacy of Rome in the Catholic Church.

A viewing perch gives visitors a close-up look at those huge letters and a heavenly perspective into the church. From the rooftop you can size up the dome you’re about to climb. To fully experience Michelangelo’s dome-within-a-dome design, climb 300 steps to the cupola.

The view from the top is unrivaled: both of Rome in general, and the Vatican grounds. You can survey the entire country from this perch. The long rectangular building is the Vatican Museum. With the adjacent Sistine Chapel, it holds perhaps the richest collection of Western art anywhere.