So be it, Lord; Thy throne shall never,
Like earth’s proud empires, pass away:
Thy kingdom stands, and grows forever,
Till all Thy creatures own Thy sway.
We quite often refer to God as king and to God’s kingdom or the kingdom of God, but we almost never refer to God as emperor or to God’s empire or the empire of God, even though King of kings and Lord of lords is more analogous to an earthly emperor than a king.
The only reference to empire/emperor/imperial in the King James/Authorised version of the bible is in the comparatively late OT book of Esther (1:20):
When the king’s decree which he will make is proclaimed throughout all his empire (for it is great), all wives will honor their husbands, both great and small.
Of the other 27 translations on Bible Hub, one uses realm, four use empire and the rest kingdom.
There are three issues: what Ahasuerus and comparable people were called in their own languages at the time (and whether those languages even had the distinction between a king and an emperor), what they are called in the relevant biblical languages, and how those biblical words are translated into English.
The bible retains Pharaoh (Egyptian), Candace (“Ethiopian”) and Caesar (Roman); all other rulers are called simply king or queen. Strictly speaking, no-one was an emperor before Roman times, anyway. (Candace and Caesar/Cesar/Cesare have become given names. I’m not aware that Pharaoh has.) The Wikipedia article on the Achaemenid (Persian) Empire gives the words xšāyaϑiya for king and xšāyaϑiya xšāyaϑiyānām for king of kings.
I couldn’t remember reading or hearing any reference to God’s empire or the empire of God. Searching for God’s empire online found God’s Empire: Religion and Colonialism in the British World, c.1801–1908 by Hilary Carey, who also wrote Empire of Hell: Religion and the Campaign to End Convict Transportation in the British Empire, 1788–1875.
Searching for empire of God found Jesus and the Empire of God: Royal Language and Imperial Ideology in the Gospel of Mark by Margaret Froelich, a blog post The Empire of God and the Postcolonial Era by Stephen D Moore (“In common with a small but growing number of interpreters, I hold that the Greek term basileia in Mark, as in other early Christian texts, is at present better rendered in English by the defamiliarizing term “empire” than by “kingdom,” a term whose political edge has been rubbed smooth by centuries of theological usage.”) and a church in Burton upon Trent.
(See also God Emperor of Dune by Frank Herbert. (A flatmate who was studying German had the translation Der Wüstenplanet, which doesn’t have quite the same ring. Maybe Wüst would be closer in feel to the original.))
I suspect that for biblical writers, kings and kingdoms (by whatever name) were sometimes good and sometimes bad and emperors and empires (ditto) were always bad. But note Isaiah 45:1 “The LORD has chosen Cyrus [of Persia] to be king!”
Part of the problem is referring to God in any human terms (king, shepherd and father spring to mind), because we will (or at least might) project the limitations of those terms onto God: kings and fathers grow old and die, and shepherds are sometimes struck and sheep of the flock scattered. Feminists also decry the masculinity of most of the terms. But we are human, and speak human languages, so must refer to God in human terms. Otherwise we won’t (or can’t) say anything at all.
A few months ago our prayer leader prayed that God would protect us from “theological empire building”. There are 10 results for “theological empire building” (in quotation marks for an exact match) online, three of which are actual usages and seven are (probably auto-generated) gibberish. References to empire building (no quotation marks) include home builders, corporate governance and a famous tower in New York, but very little about religion. References to kingdom building include home builders, a board/role playing game and a series of young adult novels, but again very little about religion.
Praying for protection against “theological empire building” is ambiguous as to whose empire building we have in mind: ours or other people’s. But surely this is something that we would never do – only they!