The reverse Chi-Rho symbol on a column in the Lateran Baptistry. Pen on paper

A mysterious symbol

inside the oldest baptistery in Rome

which seems to point back to a long-buried doctrine of the dual nature of Christ, and signal the time of year at which this transformation from man to god takes place.

Sunray entering the Lateran Baptistry. Pen on paper

by Justin Bradshaw
Architects throughout ancient times were intimately familiar with the use of sunlight, and not only for practical purposes; the Sun was regarded as the prime energy source of the Universe, the soul of all creation, and any artificial structure built by man had to be placed within the harmonies of the Universe if it was to be anything other than an aberration. This was especially true for religious structures, which were intended as a space where the divine and the earthly could interact. During the fourth and fifth centuries the baptistery of San Giovanni in Laterano was the only baptistery in Rome, and baptisms were held almost exclusively during the Easter vigil. The initiates were generally adults and underwent full immersion in the bronze bath at the centre of the building. The ceremony was an echo of the transformation of Christ, the former mortal life of the baptized would die underwater and they would be reborn with an eternal Christian soul.
This building, then, is where the divine spirit of god descends to unite with the human soul, just as the rays of sunlight descend from the Sun.

The symbol in question is the monogram of Christ, the Chi-Rho. Formed by superimposing the first and the second letter of 'Χριστός' (Khristós), the Greek term for Messiah, or 'anointed one', this image was used since at least the fourth century as Christ's monogram. This symbol can be found high up on the capitals of each of the
eight porphyry columns which surround the central baptismal font.

Eight signs of Christ, but with one significant exception; the column on
the north side of the baptistery is decorated with a mirror image of
the same symbol, an inverted Chi-Rho. The rich significance of
this inversion is revealed at the spring equinox of each year,
when a ray of sunlight enters the building at midday from
above, striking the column at exactly the right height
to form a brightly illuminated disc around the
inverted Chi-Rho.

The first baptistery on the site is thought to
have been built under Emperor Constantine,
though the columns were part of the
extensive reconstruction carried
out by Pope Sixtus III in the
430s a.d. The period was

by bitter
controversies in
church doctrine, the
Council of Ephesus was
held in 431 principally to
resolve the dispute over the
nature of Christ. The Patriarch
of Constantinople, Nestorius, held
that two separate entities coexisted
in one person, the human Jesus and the
divine Christ, with some theories adding
that the divine being entered the body of Jesus
either at his baptism by Saint John or even at the
moment of his death and resurrection. Nestorianism
had a wide following, Pope Sixtus himself had been
accused of leaning towards the doctrine, and Pope Leo
who succeeded Sixtus, complained of the way the
faithful of Saint Peter's stood on the steps and
prayed to the sun, practically an accusation
of Sun-worship which, by extension, must have
been also aimed at his predecessor. Nestorianism
was opposed by the more powerful churches,
headed by Cyril of Alexandria, and declared a
heresy in the final declarations of the Council,
Christ was declared one and inseparable from
the beginning of time.

The reverse Chi-Rho and the use of sunlight in the baptistery of San Giovanni in Laterano can be understood as a remnant of this popular but recently condemned doctrine: the disc of light would have fallen on the reverse Chi-Rho on the day of the Spring Equinox, 21st March (Easter and the Resurrection of Christ are directly connected to the Spring equinox, being fixed on the first sunday after the first full moon that follows the equinox), and it would have illuminated the monogram every day at midday until the 25th March, the Feast of the Annunciation, or the Incarnation of Christ, after which it would have begun to fall lower down on the column, leaving the symbol in the shadows. So the reversed Chi-Rho, symbolizing the opposite natures of Christ, who died as a man and became God, is bathed in a divine light until he again takes human form in his mother's womb on the day of the Annunciation.

Plan view of the Lateran Baptistery. Pen on paper
Footnote: Over the years the structure of the Baptistery has remained much the same, but the interior decorations have changed many times, and the roof was replaced at least once, in the 16th century under Pope Paul III. As a result of this restoration, the height of the ceiling and roof was raised, including the windows through which the Sun shines, meaning that today the light from the sun at the equinox falls a metre above the symbol. A study of sketches made before this restoration give a good indication of the position of the windows before the height of the roof was raised. The window from which the Sun shines would have been lower by about one metre, which means that the reverse Chi-Rho would have been illuminated at exactly the period of Easter.