Chi-Rho

 

Do you see the three P's?  Three for Trinity.  The P is Greek, Rho.  The "X" is Christ's cross.  Thus this symbol is called Chi-Rho or a Christogram, common in the early centuries of Christianity. This symbol is on the ceiling of the baptistry of Albenga.  Around the monogram are 12 doves for the 12 apostles.

This is an important piece of architecture in northern Italy. As people were baptized they were underneath this dome.  Imagine looking up.  There is symbolism everywhere.  Remember that most people couldn't read and were taught their catechism from architecture and stained-glass windows, and other visualizations of Christianity.