In the wake of Julius Caesar's assassination, his political allies in the aristocracy are in desperate need of legitimacy to fight Caesar's assassins. Deification of Caesar is one of their tools: when Caesar becomes Divus Iulius, his heir and nephew Octavian becomes Divi filius ("Son of God").
As justification, Octavian claimed to have witnessed a bright comet in the sky shortly after the assassination and believed that this comet was actually Caesar's soul being received as an equal among the gods. Julius Caesar thus becomes the very first Roman citizen to ever be named a god. He is also the first Roman citizen to have his own temple.
The Temple of Caesar or Temple of Divus Iulius is started now, but it won't be completed until 29 BCE.
