"Dr. Van Til taught us that "There is no alternative but that of theonomy and autonomy" (Christian-Theistic Ethics, p.134). Every ethical decision assumes some final authority or standard, and that will either be self-law (autonomy) or God's law (theonomy). While unbelievers consider themselves the ultimate authority in determining moral right or wrong, believers acknowledge that God alone has that position and prerogative.
The position which has come to be labeled "theonomy" today thus holds that the word of the Lord is the sole, supreme, and unchallengeable standard for the actions and attitudes of all men in all areas of life. Our obligation to keep God's commands cannot be judged by any extrascriptural standard, such as whether its specific requirements (when properly interpreted) are congenial to past traditions or modern feelings and practices.
The law defines what sin is (1John 3:4). As such the law cannot be our personal vehicle for gaining favor with God. It rather aims at Christ as our only righteousness, tutoring us that justification must be by faith in Him (Rom 10:4; Gal 3:24).
So theonomy teaches that since the fall it has always been unlawful to use the law of God in hopes of establishing one's own personal merit and justification....." (Greg Bahnsen)