This image shows the interior of the dome of St. Peters.
The dome is at the "crossing" of St. Peter's nave, and built over what is universally believed to be the tomb of St. Peter.
At the base of the dome, is a portion of the Biblical inscription in Latin, (seen here in gold mosaic), from Matthew 16:18-19
where Christ gives the keys of the kingdom of heaven to Peter:


"TV ES PETRVS ET SVPER HANC PETRAM AEDIFICABO ECCLESIAM MEAM. TIBI DABO CLAVES REGNI CAELORVM"

These letters are 8 ft high (2.5 m), and translated:
"You are Peter, and on this rock I will build my church.... I will give you the keys of the kingdom of heaven"