A few steps away from the Cathedral of St Dominus and St Duje -- at the end of the street Kraj Sveti Ivan -- is a temple dedicated to Jupiter, named after his father. Roman emperors often made themselves a god. Diocletian was Jovius, son of the top god, Jupiter. This god was highly worshipped during the Imperial era until the Roman Empire came under Christian rule. Emperor Diocletian believed he was a reincarnation of Jupiter and thus positioned this temple directly adjacent to his mausoleum, not St Dominus Cathedral.
When Diocletian's mausoleum became a cathedral, the temple was converted into a baptistery, housing a huge 12th-century baptismal font large enough to immerse someone (as was the tradition in those days). Jupiter is considered to be one of the best-preserved Roman temples in the world.
The temple once had a porch supported by columns, but the one column you see dates from the 5th century. The headless sphinx in black granite guarding the entrance was imported from Egypt at the time of the temple's construction in the 5th century. The walls of the temple support a barrel-vaulted ceiling and there's a decorative frieze around the other three walls. Directly next to the temple is Split narrowest street called "Pusti me proć," meaning let me through. Below the temple is a crypt, which was once used as a church, and is dedicated to St. Thomas.
Don't forget to explore the doorway to the temple, which features very intricate moldings.