The Cathedral of St John Lateran (S. Giovanni in Laterano) is certainly one of the most ancient and most important of the Christianity after St. Pietro, because the principal Cathedral in Rome.
There is the altar on which only the pope, bishop of Rome, can celebrate the Mass. As in every greater basilica, the entry is crossed by a great portico where you find the statue of Costantino and five doors, the right one is the Holy Door, that opens only for the Holy Year.
The façade of S. Giovanni in Laterano is a late baroque masterpiece realized by Alessandro Galilei in 1732, overhung by 15 colossal statues, while the inside is of five aisles, suggestive and scenografic and was restructured in 1600 by Francesco Borromini. You can see an admirable Tabernacle built by of Giovanni di Stefano. From the left you can access to the extraordinary Cloister, true masterpiece of the thirteenth-century build by Vassalleto.
Next to the church the ancient Baptistery, erected by Costantino together with the basilica, which has an octagonal form and in the middle an urn of green basalt, once baptismal source. Absolutely, not to miss, you can see the inestimable relics of the Cross of Christ inside the church Santa Croce in Gerusalemme, near Porta Maggiore and the Holy Stairs (Scala Santa).