God’s truth is not arbitrary: His revelation and His natural law have one and the same source. As Christians, we may – no, must – act on what we know to be the truth; meanwhile, to them that do not recognize revelation as a source of truth we must argue rationally on the lines of natural law, drawing upon the same nature of the one true God who created and ordered the cosmos. In short, truth is truth, by whatever means we know it. We are duty-bound to act by the truth, because there is no neutrality between truth and falsehood.
So how do we get from truth to decision? What we consider in decisions of public policy is not where God is and is not Lord: that point is settled, that God is Lord over all spheres of life. Rather what we must consider is what the temporal government’s authority is as instituted by God and constituted according to His order and how well a possible choice fulfils that charge, while we take care in making policy to ensure that government does not overstep the authority given by God.
Simple enough to formulate in the abstract as an autonomous principle, and thankfully so, because it’s much messier when implementation requires thought about many related principles.