I’m not getting where “the idea of theocracy is… common to… Christianity.” My understanding, incomplete though it must be, is that Christianity has usually been implemented alongside of more-or-less secular governments. Within the Roman Empire, it was mostly illegal to be a Christian until after the early 4th century. It is true that, as the Roman Empire dissolved, the Papacy increasingly filled the void, as a matter of necessity, rather than as a precept of religion. In later centuries, this heavy involvement of papacy with government was proven unwise, and by the mid-19th century became a thing of the past, blessedly so.
Anyways, I do not see from history, or from Church teachings, an urge to theocracy. In fact, I see just the opposite. Jesus said, render unto Ceasar what is Ceasar’s, and unto God what is God’s. I take it that Jesus was referring to a larger question than mere taxation by giving that reply.