Theonomy boils down to this statement, made by an old friend:
If Christ is God, and if Christ is Lord, then His Lordship extends to all areas of inquiry: metaphysics, epistemology and ethics. And so, the assumption, if one affirms ethical absolutes, is necessarily one of “theonomy” broadly construed in opposition to autonomy, as Cornelius Van Til indicated. Therefore, the question is whether one is a “consistent” theonomist or an “inconsistent” theonomist.
…one better presuppose a theonomic ethic (in a broad sense not necessarily ala Bahnsen), or otherwise, one is left without ethics, and therefore, without Lordship. The details of a theonomic ethic need to be determined through careful exegesis. But, what we cannot do is say that God has no claim on how we are to live – whether privately or publicly; he does have a claim, and that claim is a theonomic (God’s law) claim.
And, given Romans 13 and other passages, the notion of justice is never abstracted from God and His character, even if public justice is in view. Consequently, public justice exhibits a theonomic dimension.