The idea that the Old Testament retains its authority in certain select areas (e.g., family, and church) in this age, but not in others (e.g., the state) is foreign to the New Testament. The canonical Scriptures of the Old Testament contain extensive “instruction in righteousness” for both magistrates and citizens. The teaching of Christ and the Apostles, and the statements of the confessions of the church affirm that this instruction continues as part of the “rule of faith and life.”
Therefore, the doctrine of the full inspiration and authority of both Testaments provides a potent argument for the authority of the Old Testament in the state. If God has spoken to the issues of civil government in the Old Testament (e.g., the duty of magistrates to judge justly, Deut. 16:18-19), then we must grant that word the same divine authority to instruct us in righteousness for that sphere as we grant it, say, for the sphere of family (e.g., parental instruction in discipline, Prov. 23:13). The authority of God’s revelation in the Bible is seamless. The Christian view of Scripture is: “The Bible is thought of as authoritative on everything of which it speaks. And it speaks of everything.”
Einwechter, W. (1998). The Authority of the Old Testament in the State. Darash Press. Available http://www.darashpress.com/articles/authority-old-testament-state. Last accessed 14 Mar 2016.