I saw the fresco below a few years ago when I went to the British Museum in London. It is from a Roman-era villa in Britain, from the 3rd/4th century AD.
The symbolism is fascinating to me:
– In the very middle is the Chi Rho, which is the monogram of Christ, being the first two letters of the name in Greek
– On the left side of the Chi Rho is the letter Alpha, and on the right the letter Omega. Christ refers to himself as “the alpha and the omega, the beginning and the end.”
– The letters themselves are in a circular structure which appears to be floating on water, with a dove on the lower left. This appears to represent Noah’s Ark and, possibly, the rainbow given by God as a sign. Now, however, the rainbow is complete (i.e. 360 degrees as seen in the book of Revelation), and Christ is the Ark. This symbolizes both Christian baptism and the Church. That is, Christ is the one who carries us across the waters of baptism, saving us, and brings us into the saving Church.
(Image by I, Udimu, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2461548