"Ye have heard how I said unto you, I go away, and come again unto you. If ye loved me, ye would rejoice, because I said, I go unto the father: for my Father is greater than I. And now I have told you before it come to pass, that, when it is come to pass, ye might believe." (John 14:28-29, emphasis mine)
So, theonomy is one of those words from your Theology textbook. A lesser-known cousin by virtue of it's prefix, it's essentially a bottom-up theocracy. Going forward, I will attempt to define it under the context of the free-form and organic "Body of Christ". Because until Christ returns, as He mentions above, we have only the Holy Spirit to guide our actions and thoughts in and among one another—even as we walk in our own autonomy. Our own autonomy, provided we don't hurt one another. And this is where, for us to truly live as a coherent Body, we need the Holy Spirit.
"Follow peace with all men, and holiness, without which no man shall see the Lord. Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled;" (Hebrews 12:14-15)
I can see it now. Critics to a theocratic way of life might cite the straw-man example of your garden variety, polyester three-piece suit clad preacher taking the stage and mouthing some words prior to rifling through your pocketbook. The trouble with this kind of pointed thinking is that this person genuinely does exist, they most-likely are indeed born again, and yet are just as in need of forgiveness and love and prayer as any brother or sister in Christ. A "spiritually theonomic" (forgive me for coining this phrase, it's essentially redundant) mindset takes into account that the only person from whom we take all our instruction—is Jesus Himself. And when believers in Christ truly don't know Him (this is a thing), pandemonium ensues and the collusion and confusion alluded to in the previous part will fill in like a flood. Ready for someone to take the stage and sort it all out. But!
"So shall they fear the name of the Lord from the west, and His glory from the rising of the sun. When the enemy shall come in like a flood, the Spirit of the Lord shall lift up a standard against Him. And the Redeemer shall come to Zion, and unto them that turn from transgression in Jacob, saith the Lord. As for me, this is my covenant with them, saith the Lord; My spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth..." (Isaiah 59:19-21a)
It's referring to Jesus, I should just like to point that out right here.