As you may or may not know, the idea of Indo-European (or Aryan before the Nazis abused the term) arose when someone noticed some strong linguistic similarities between Sanskrit and Latin. Once the connection was made many European languages and several over the Middle East and into India were similarly connected to a race whose language was the forerunner of them all.
But it also opened up the possibility of other connections for many of the cultures between Ireland and India, among them the original Indo-European religion. And primary among their pantheon of gods was Dyeus Phter, controller of storms who worked from the mountains. I don’t want to bore anyone with the subtleties of linguistics (nor am I qualified to get into a lengthy discussion), but the basics are this: As the core of the Celtic, Nordic, Romance, Greek, Indic, and Persian language groups were developing they made subtle changes to their vocabulary and this translated to the deities they worshipped. He became Zeus among the Greeks, Thor and Tyr for the Norse, Dies Piter with the Romans, Dyaus Pita for the Indians, and the Illyrians called him Dei Patrous. The Indo-Iranians, Baltic, Celtic, and Slavic peoples would worhship him as Deva/Daevas, Dievas, Deuos, and Div respectively.
But it’s also clear the nature of the god changed along with his name. Though he existed among the Indo-Europeans as the father of the gods (“Dyeus” meaning god and “Pater” father) he did not retain that position unilaterally. Part of the reason for that might have been because of the cultures the Indo-Europeans conquered after they had separated. Other reasons might have been their new environment or their varied experiences as distinct cultures. The Norse for instance would retain Thor’s attributes but would soften his personality into the protector of mankind and something of a bumpkin while Odin would become the dominant god. He seems to have no linguistic relative anywhere, but his name is an indicator of mystical wisdom.
What is clear is that Dyeus Pater began his existence as a storm god. A culture’s deities tend to reflect the culture they came from which means that the Indo-Europeans of pre-3000 B.C.E. felt very much at the mercy of the elements and hoped that by worshipping the strongest god, the god of lightning, they might give themselves a better chance of survival. This changed as everyday survival became more certain; the sun would come to symbolize prosperity and health, the sea would remain inconsistent but would gain importance as the Indo-Europeans developed naval trading. The forge, wine, and war would all come into their own in the following centuries. The cultures they conquered would additionally give them a mother goddess, fertility, and respect for the moon. It was with the storm-god, though, that the Indo-Europeans had their core.