Here's what "A dictionary of selected synonyms in the principle Indo-European languages" has to say:
OE god, os
St. deva-, sura-
For 'god' there is a group of cognates common to Italic, Celtic, Baltic, and Indo-Iranian (traces in Gmc. , but not the usual word for 'god'), related to words for 'sky', 'day', and the widespread 'Sky-god', all from then otion of 'bright, shining'. A smaller group, common to Slavic and Iranian, is based on the notion of 'one who dispenses, gracious'. The other words are of disputed etymology.
The old words for a pagan 'god' were generally retained for the Christian 'God'. But a few forms are used only in the former sense.
1. EI *deiwo-s in words for 'god', beside *dyew- *diw- in words for 'sky', 'day' and the personified Grk. Zeus, gen Dios, Lat Iuppiter (earlier Iupiter, fr. voc. = GRk Zeu pater) gen. Iovis, early Diovis, Skt. dyaus, all with the common notion of 'bright, shining' and representing an extension of the simpler *dei- seen in Skt. dideti 'shines', etc. ...
OLat deiuos, Lat. deus (> It. dio, Fr. dieu, Sp. dios; Rum zau interj., zeu 'pagan god'; but for Christian God dumnezeu, fr. Lat. voc. domine deus 'Lord God'), Osc fem. dat. sg. deivai; Ir. dia, W. duw, Br. doue; ON tivar (pl.; cf. ON Tyr, OE Tig, gen. Tiwes, OHG Zio); Lith dievas, Lett. dieus, OPruss deiws; Skt. deva- (Av. daeva, OPers. daiva- 'demon)
2. ChSl. bogu, etc., general Slavid (per. early loanword fr. Iran. through the Scythians; cf. the Slavic word for 'dog'), Av. baga, OPers. baga-: Skt. bhaga- 'dispenser, gracious lord', bhaj-'divide, distribute, share', Grk. aor. phagein ('partake of' >) 'eat'.
3. Grk theos, fr *Thesos (cf. thesphatos 'spoken by god, ordained'), but root connection much disputed and still dubious. Perh. best (but difficulties): Lat. (dies) festus 'holiday', feriae 'holidays', Osc. fiisnam, Lat. fanum (*fas-no-) 'shrine', fr. *dhes-, *dhes-, prob. an extension of *dhe- 'put' in its frequently attested religious application.... Mrs Hopkins, op cit, rejects all the proposed IE etymologies and suggests that, like some of the names of the Greek gods, so theos itself is a loanword from pre-Greek sources.
4. [germanic languages]
5. Skt. sura- , abstracted fr. asura- (as if a-sura-) after this had come to mean 'evil spirit' vs. earlier sense 'spiritual beneficent spirit' (in RV freq. epithet of the gods, esp. Varuna; = Av. ahura mostly in Ahuro Mazda), prob asu-ra-, fr. asu- 'breath of life, life'.