Why is the idea of dualism so threatening to many contemporary Reformed Christians? To talk about two kingdoms or to introduce the idea of differences between sacred and common jurisdictions is apparently a concession to secularism and a denial of Christ’s Lordship over every square inch of created order.

But in point of fact, a streak of dualism runs through the Reformed tradition. To help the medicine go down, the following quotations may show that dualism is less scary that it first seemed.

Therefore, to perceive more clearly how far the mind can proceed in any matter according to the degree of its ability, we must here set forth a distinction: that there is one kind of understanding of earthly things; another of heavenly. I call “earthly things” those which do not pertain to God or his Kingdom, to true justice, or to the blessedness of the future life; but which have their significance and relationship with regard to the present life and are, in a sense, confined within its bounds. I call “heavenly things” the pure knowledge of God, the nature of true righteousness, and the mysteries of the Heavenly Kingdom. The first class includes government, household management, all mechanical skills, and the liberal arts. In the second are the knowledge of God and of his will, and the rule by which we conform our lives to it. John Calvin, Institutes, II.2.13

. . . you cannot expect from a true Christian church any official pronouncements upon the political or social questions of the day, and you cannot expect cooperation with the state in anything involving the use of force. Important are the functions of the police, and members of the church, either individually or in such special associations as they may choose to form, should aid the police in every lawful way in the exercise of those functions. But the function of the church in its corporate capacity is of an entirely different kind. Its weapons against evil are spiritual, not carnal; and by becoming a political lobby, through the advocacy of political measures whether good or bad, the church is turning aside from its proper mission. . . J. Gresham Machen, “The Responsibility of the Church in the New Age” (1933)

And just in case the biblicists are still holding out, consider 2 Cor 4: 16-18:

Therefore we do not lose heart. Though outwardly we are wasting away, yet inwardly we are being renewed day by day. For our light and momentary troubles are achieving for us an eternal glory that far outweighs them all. So we fix our eyes not on what is seen, but on what is unseen. For what is seen is temporary, but what is unseen is eternal.