Diocletian was one of the last emperors to worship traditional Roman gods; within a half-century, Christianity would become the official religion of Rome. 

Translated, the obverse reads: "Emperor Cesar Valerius Diocletian Patriot Augustus." The reverse reads: "Jupiter and Hercules Preservers (of the) Augustus Cesars."  At the time, two emperors—Diocletian and Maximian—each ruled half the Roman empire, each with a subordinate Cesar. Diocletian fancied himself as the god Jupiter, and Maximian as Hercules; as such, they could "protect" their Cesars. 

This is a very telling and rare Roman coin.