The Greek is “Διὸς…βουλή” where “βουλή” means “will” and “Διὸς” is the genitive of “Ζεύς”, hence “will of Zeus”.
So why did Rouse translate “Zeus” as “God” here? This is clearly a deliberate strategy and not a mistake (as suggested by the other answer), because Rouse makes this choice in many places in the text. Here are three examples, comparing Rouse’s translation to the 1924 translation of A. T. Murray:
I.63καὶγάρτ᾽ὄναρἐκΔιόςἐστιν,
[Murray] for a dream too is from Zeus
[Rouse] for God, it seems, doth send our dreams
I.85‘θαρσήσαςμάλαεἰπὲθεοπρόπιονὅτιοἶσθα:
[Murray] Take heart, and speak out whatever oracle you know
[Rouse] Fear nothing, but speak the word of God which you know.
II.205εἷςβασιλεύς, ᾧδῶκεΚρόνουπάϊςἀγκυλομήτεω
[Murray] one king, to whom the son of crooked-counselling Cronos [that is, Zeus]
[Rouse] One king’s enough for me, and why? He gets the right from God on high!
What I think Rouse had in mind, is that there is a risk in translating the Iliad, of allowing the modern reader to interpret the Homeric gods in a very different way from the ancient audience. The modern reader is likely to take the gods as characters in a story, like the heroes but with extra magic, missing the divinity and reverence they were accorded by Homer’s audience. So Rouse uses “God” to translate various religious concepts—“Zeus”, “oracle” and “son of Cronos” in the three examples I gave above—in an attempt to convey to a modern, Christian, reader the piety of the speaker in the poem.
