The Baptistery of San Giovanni in Fonte
The oldest cathedral in Naples is said to be Santa Restituta (see that link), now incorporated as a paleo-Christian “church within a church” in the present-day Naples cathedral—or Duomo. Within Santa Restituta, however, is a baptistery described by literature about the site as the oldest one in Western Christendom. The construction of S. Restituta and baptistery goes back to the time of the emperor Constantine the Great (280-337 AD); this is attested to by a passage from the life of Pope Silvester I in the Liber Pontificalis Ecclesiae Romanae: “[…]eodem tempore fecit Costantinus Augustus basilicam in civitatem Neapolim" (…at that time, Costantinus Augustus had a basilica built in the city of Naples). Reliable archaeology places the construction in the fourth century with the first modifications done in the fifth century.
Once inside the main cathedral, the entrance to Santa Restituta is on the left, past the fourth chapel; you enter and are in the back of this church within a church, facing the apse. On the left of the apse is a stairway down to Roman and Greek remnants of ancient Neapolis beneath the Duomo; to the right is a doorway into the baptistery, itself. The entire baptistery consists of two chambers of unequal size separated by columns. The larger of the two is the one of interest and is the first one you enter from the main body of S. Restituta. It is a square chamber 7.60 meters (25 feet) on a side. Starting well above eye-level, the walls then create an octagonal base that culminates in a dome directly above the baptismal font itself, in this case a sunken bath-sized tub large enough for the rites of immersion.
Because of their age, the mosaics within the baptistery are of considerable interest to Christians—indeed, to any historian of religion—and there is a considerable body of literature on them. The mosaics start at the center of the dome, above the font, with a large Christological monogram; that is, a stylized rendering of the first two Greek letters of the name of Christ—X and P (the sounds ch and r). This monogram of the name of Christ is called the Chrismon. In this rendition, it is flanked by the Greek letters alpha and omega, the first and last letters of the Greek alphabet, a reference to the book of Revelation, chapter 1, verse 11 (KJV): "I am Alpha and Omega, the first and the last." That is then surrounded by other mosaics that flare out and down to the octagonal base of the dome. Among the other mosaics, most of which are fragmentary:
Also, there is the mosaic illustrated
(above)* in this entry. It juxtaposes two episodes in
the life of Christ: one, His encounter with the Samaritan at the well;
two, the miraculous
changing of water to wine at the Wedding at
Cana. The first refers to the fourth chapter (KJV) of
the Gospel of John:
 Jesus answered and said unto her, Whosoever drinketh of this water shall thirst again:The second reference is to the miraculous changing of water to wine at the wedding feast at Cana, from the second chapter (KJV) of John:
 But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.
 And the third day there was a marriage in Cana of Galilee...The baptistery and mosaics have been recently restored. This evokes two conflicting schools of thought: one, restoration, as far as possible, to the original state; that is, recreate the splendid view that greeted the baptized as they stared up at the monogram of the name of Christ, itself surrounded by immaculate and detailed symbols of their faith; two, preserve the current fragmentary state of the mosaics and keep them from deteriorating further. The restorers have chosen the second route. Anything else, they say, would be to create a counterfeit. I have no opinion on this except to note that most antiquity could not be viewed at all today if someone had not put at least some of the "original pieces" back in place. You don't look at the temples in Paestum, for example, and think, "Gee, too bad they restored these." However, it is also true that neither those temples, nor the ruins of Pompeii and Herculaneum look as they did two-thousand years ago. They have been frozen in a state of well-maintained decay. Works of art, on the other hand, present a different problem. Michelangelo's art in the Sistine Chapel was recently restored and is said to look the way it did when it was created. These mosaics may be yet another problem. I'm glad I don't have to decide.
 And both Jesus was called, and his disciples, to the marriage.
 And when they wanted wine...
 Jesus saith unto them, Fill the waterpots with water...
 And he saith unto them, Draw out now, and bear unto the governor of the feast...
 ...the ruler of the feast...tasted the water that was made wine...
 This beginning of miracles did Jesus in Cana...and manifested forth his glory...
My thanks to Fr. Ilya Gotlinsky for reminding me of this site.
*The photo is a cropped version of a photo credited to Giusy Mennillo. I have not obtained permission to reproduce it because I have been unable to contact the copyright holder. I will keep trying and will gladly remedy that situation if someone provides me with information, or I will remove the photo upon request.
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