Julius Caesar is deified by his political allies in Rome. In the wake of Caesar's assassination, his aristocratic allies desperately need 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 claims to have witnessed a bright comet in the sky shortly after the assassination and says 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.
Construction of the Temple of Caesar or Temple of Divus Iulius starts immediately, but it won't be completed until 29 BCE.