The Rigveda and the Gathas are close enough that a reader of one can often make out the other, and the correspondence extends to the vocabulary of ritual, of sacrifice and of the divine. The plant pressed and offered in the Vedic rite is soma; in the Avestan rite it is haoma, the same word. The Vedic fire priest is the hotr; the Avestan is the zaotar. This is the strongest linguistic evidence anywhere for a common ancestral religion behind two attested traditions, and it is the model on which comparative Indo-European religion has been built. The famous complication is a reversal of value. The Vedic word deva names a god, and its Avestan cognate daeva names a demon.
The Vedic asura is ambiguous and often hostile, while the Avestan ahura, in Ahura Mazda, names the supreme being. Both sides use the same inherited vocabulary and assign it opposite polarity. What happened has been argued for a century and a half. Reforming movement, priestly schism, ordinary semantic drift and a scholarly artefact of the sources have all been proposed. The atlas holds the correspondence as established and the explanation as open, because that is the honest position. It is worth adding that the reversal is not total. Mithra and Mitra, Yima and Yama, Verethragna and Vritrahan all correspond without inversion, so whatever happened operated on a particular class of terms rather than across the board, and any explanation has to account for that selectivity as well as for the reversal itself.