Therefore, the classic Reformed theological paradigm suggests that Christians are citizens of two distinct kingdoms, both of which are ordained of God and under his law, yet exist for different purposes, have different functions, and operate according to different rules. In their capacity as citizens of the spiritual kingdom of Christ, Christians insist upon non-violence and the ways of peace, refusing to bear arms on behalf of his kingdom; in their capacity as citizens of the civil kingdom, they participate as necessary in the coercive work of the state, bearing arms on its behalf when occasion warrants. As citizens of the spiritual kingdom they have no patriotic allegiance to any earthly nation; but as citizens of the civil kingdom a healthy patriotism is certainly possible. As citizens of the spiritual kingdom they can make radical critiques of all theories, practices, and institutions that are not submissive to the redemptive lordship of Christ; but as citizens of the civil kingdom they can acknowledge the significant benefits that the state brings for earthly life, enjoy the amazing products of human culture, and seek common cause with non-Christians on a variety of social projects. As citizens of the spiritual kingdom they submit to the redemptive ethic of Scripture; yet as citizens of the civil kingdom they can engage in genuine moral conversation with those of other faiths through the universally accessible law of nature, without making adherence to Scripture a test for participating in cultural affairs. As citizens of the spiritual kingdom they can view the state and other social institutions as temporal and destined to pass away; but as citizens of the civil kingdom they can have a keen interest in promoting the welfare of human society here and now.
David VanDrunen, Natural Law and the Two Kingdoms: A Study in the Development of Reformed Social Thought (pgs. 13-14)