The Via Francigena dates to the eleventh century. It is an ancient pilgrimage route between Canterbury, England, and Rome, and which also passes through France and Switzerland. The Seven Pilgrim Churches were often visited at the terminus of the journey. In fact, it was considered obligatory to visit those four Pilgrim Churches of Rome that are major basilicas. The two most important are St Peters Basilica in the Vatican (San Pietro in Vaticano) and San Paolo Fuori Le Mura (St. Paul Outside the Walls). The other two that are considered obligatory at the end of a pilgrimage are San Giovanni in Laterano (St. John Lateran) and Santa Maria Maggiore (Sainty Mary Major).
The other of the ancient Pilgrim Churches of Rome (the minor basilicas) are: Saint Lawrence Outside the Walls (San Lorenzo fuori le Mura), Saint Sebastian Outside the Walls (San Sebastiano fuori le Mura), and the Basilica of the Holy Cross (Santa Croce in Gerusalemme). A more recent addition is the Our Lady of Divine Love (Santuario della Madonna del Divino Amore), added by Pope John Paul II for the 2000 Jubilee celebration, and replacing Saint Sebastian Outside the Walls. Most just add this to their list of Seven Pilgrim Churches to visit, meaning they will visit eight.
Numerous Rome tour companies and international religiously oriented travel companies offer Seven Churches tours on their itineraries, and these church visits are as important to devout Catholics as the haj (pilgrimage) to Medina is to Muslims. There are also Seven Churches tours in Turkey that visit all seven of the Christian churches mentioned in the Book of Revelations, including the church at the extraordinary ancient city of Ephesus. If you are dealing with a travel agent, be sure to specify the Pilgrim Churches of Rome so you don't end up in the wrong country!
St Peters Basilica within Vatican City is undoubtedly the most famous of the Seven Pilgrim Churches and almost all visitors to Rome will pay a visit here. Inside is a statue of St. Peter that has one foot rubbed completely smooth from centuries of touching and kissing by pilgrims. Also within the basilica is the Pieta Chapel, containing possibly the most famous of all religious sculptures. This Pieta sculpture is the earliest on this theme by Michelangelo, who also painted the beautiful Sistine Chapel ceiling. Beneath the high altar is what is said to be the tomb of St. Peter, and the great square outside is one of the sites ("Air" of the four elements) in the best-selling novel by Dan Brown and film starring Tom Hanks (Angels and Demons). Another of the sites in the film and novel is Bernini's famous Fountain of Four Rivers in Piazza Navona. If you make arrangements for your Seven Churches tours in advance, it is possible to additionally tour the Scavi, the underground necropolis beneath the basilica.
As its name implies, the Basilica of San Paolo Fuori Le Mura is located outside of what were the walls of the ancient city, and like St. Peter's is supposedly built over the burial site of the martyred saint. Because it was outside the walls, it suffered damage over the centuries by invaders, most significantly the Saracens in the ninth century, and has been rebuilt and enlarged many times over the centuries. It is some distance away from the dense cluster of other Rome religious attractions, but still on the main tourist routes.
San Giovanni in Laterano is dedicated to both John the Baptist and John the Evangelist. It is the first church allowed by the Emperor Constantine (in the fourth century) where Christians could openly worship. As such is called the Mother Church by Catholics, and is even more important religiously that the more famous St. Peter's. It is only a few blocks down Via de Strata Statle from the central roundabout that encircles the Roman Amphitheatre. Santa Maria Maggiore was also founded in the fourth century, and is so named because it is the largest of the 26 churches dedicated to Virgin Mary in the city. It is also the oldest church in Western Europe dedicated to the Virgin Mary.