the supreme sky god), and thus exhibit shared functions by nature. Others required more expansive theological and poetic efforts: though both Ares and Mars are war gods, Ares was a relatively minor figure in Greek religious practice and deprecated by the poets, while Mars was a father of the Roman people and a central figure of archaic Roman religion. Some deities dating to Rome's oldest religious stratum, such as Janus and Terminus, had no Greek equivalent. Other Greek divine figures, most notably Apollo, were adopted directly into Roman culture, but underwent a distinctly Roman development, as when Augustus made Apollo one of his patron deities. In the early period, Etruscan culture played an intermediary role in transmitting Greek myth and religion to the Romans, as evidenced in the linguistic transformation of Greek Heracles to Etruscan Her[e]cle to Roman Hercules. Interpretatio romana The phrase interpretatio romana was first used by the Imperial-era historian Tacitus in the Germania. Tacitus reports that in a sacred grove of the Nahanarvali, "a priest adorned as a woman presides, but they commemorate gods who in Roman terms (interpretatione romana) are Castor and Pollux." Elsewhere, he identifies the principal god of the Germans as Mercury, perhaps referring to Wotan. Some information about the deities of the ancient Gauls (the continental Celts), who left no written literature other than inscriptions, is preserved by Greco-Roman sources under the names of Greek and Latin equivalents. A large number of Gaulish theonyms or cult titles are preserved, for instance, in association with Mars. As with some Greek and Roman divine counterparts, the perceived similarities between a Gallic and a Roman or Greek deity may reflect a common Indo-European origin. Lugus was identified with Mercury, Nodens with Mars as healer and protector, Sulis with Minerva. In some cases, however, a Gallic deity is given an interpretatio romana by means of more than one god, varying among literary texts or inscriptions. Since the religions of the Greco-Roman world were not dogmatic, and polytheism lent itself to multiplicity, the concept of "deity" was often expansive, permitting multiple and even contradictory functions within a single divinity, and overlapping powers and functions among the diverse figures of each pantheon. These tendencies extended to cross-cultural identifications. In the Eastern empire, the Anatolian storm god with his double-headed axe became Jupiter Dolichenus, a favorite cult figure among soldiers. Application to the Jewish religion Roman scholars such as Varro