May 2016 - Jupiter was the chief of the gods for the Romans, the way Zeus was for the Greeks and Thor was for the Germans. In fact, they must have all originally been the same Indo-European sky god. Jupiter and Zeus really have the same name - Jupiter comes from Ju-pater, which means "Ju-the-father" in Latin. You can see that "Ju" sounds basically the same as "Zeus". And all three of these gods use thunder and lightning as their weapons.
The Romans often thought of Jupiter as part of a group of three gods, Jupiter, Juno, and Minerva. They seem to have gotten this idea from the Etruscans. Many Etruscan and Roman temples were dedicated to all three of these gods together.
As the Romans began to trade with the Greeks and then to conquer Greek land in Italy, and finally to conquer Greece itself, the Romans became more familiar with the culture of their Greek neighbors. They saw Zeus as another form of Jupiter, and so gradually Roman ideas about Jupiter got to be more and more like Greek ideas about Zeus. By the time of Augustus, about 30 BC, Roman poets like Ovid retold many stories about Zeus as if they were also about Jupiter. And they called the biggest planet "Jupiter", because Greek astronomers had called it "Zeus" (which they did because earlier Babylonian astronomers called that planet "Marduk"). But the Romans kept on building temples to the three gods - Jupiter, Juno, and Minerva.
By the 300s AD, most people were beginning to be Christians, who didn't believe in Jupiter anymore. Even people who weren't Christian were mostly worshipping newer gods: the Unconquered Sun, or Mithra, rather than Jupiter. Still people kept on calling the planet Jupiter.