Instrumentum regni (literally, "instrument of monarchy", therefore "of government") is a Latin phrase perhaps inspired by Tacitus,[1] used to express the exploitation of religion by State or ecclesiastical polity as a means of controlling the masses, or in particular to achieve political and mundane ends.
Among the oldest and most important there was undoubtedly the Greek historian Polybius, who in his Histories says: I believe that it is the very thing which among other peoples is an object of reproach, I mean superstition, which maintains the cohesion of the Roman State.
These matters are clothed in such pomp and introduced to such an extent into their public and private life that nothing could exceed it, a fact which will surprise many.
For this reason I think, not that the ancients acted rashly and at haphazard in introducing among the people notions concerning the gods and beliefs in the terrors of hell, but that the moderns are most rash and foolish in banishing such beliefs.Before Polybius, a similar thesis was expressed in the fifth century BC.
from the Athenian politician and writer Critias, disciple of Socrates, in a satirical drama called Sisyphus, of which a long fragment has been handed down to us.