Evangelicals often use the word “gospel” as an adjective when talking about engaging the culture: “gospel justice,” “gospel prudence,” “gospel love,” etc. Though these terms are vague, they seem to indicate some significant role of the Gospel in shaping the Christian’s public and political life. The “gospel” modifies some term, forming something distinctively Christian.
While I endorse the adjectival use of the gospel in civil matters (as opposed to the claim that gospel inaugurated a radical social project), one must use it in accordance with certain distinctions. These gospel duties (let’s call them) must relate in some way to creational or natural duties.
Let’s say that there are gospel duties. Any proposed gospel duty relates to natural duties in one of four ways:
1. Same (in substance) as the natural
2. Contradictory (in substance) with the natural
3. Indifferent to nature.
4. Perfective of nature.
1. A gospel duty that is the same in substance as some natural duty is essentially the same command given through a different mode of delivery: one through nature known by reason and the other through scripture known by faith. There is nothing substantially new other than the different means of delivering the same command. (e.g., the knowledge that one ought to honor his father and mother is known by reason and faith).
2. When contradictory, the alleged gospel duty contradicts some natural duty. Since the voice of nature and God are the same, any specially revealed duty that contradicts the prior voice of God in nature is a divine contradiction. Since God cannot contradict himself (in this way at least), there is no gospel duty that contradicts a natural duty.
3. Duties indifferent to nature are divine commands about which nature has nothing directly to say. Humans are free to do what nature does not forbid, though all things must be in accordance with prudence and wisdom. (See Calvin on customs). As he did with certain civil laws of Israel, God by his divine right can bind man to laws indifferent to nature.
But any alleged gospel duty of this type must not undermine nature in some way. If their implementation serves to destroy the balance of the natural civil order, then they are not legitimate duties. For example, a Christian school might implement a policy of toleration towards unruly, vulgar, intractable, and disruptive children in the interest of showing them “grace” and the gospel. But the toleration of such behavior, being a major distraction to the disciplined children, would undermine the principal end of the Christian school: education. It undermines the balance of order in the school and thereby undermines the fulfillment of the school’s chief end.
Christians must therefore show that their alleged gospel duty does not undermine the natural order of things. Showing “grace” to the social miscreant might actually work to undermine the natural civil order. The “gospel” would then destroy nature, and Christians would then be agents of disorder.
For this reason, if there are such gospel duties, Christians must recognize that their fulfillment requires careful reasoning and application. They must be in addition to nature, not opposed to it. You cannot propose some gospel duty of this sort without clarifying its relationship to the natural order.
4. A “perfective” quality is one that is necessary for something’s perfection but is dispensable to the thing in itself. That is to say, the thing does not need it to continue being that thing. A car can still run without a shiny paint-job. The paint is inessential to the car as a car, but it is necessary for the car’s perfection. At the same time, the perfective quality is closely connected with the thing it perfects. It is not adventitious, i.e. it is not merely some addition, like a football team car flag. It is rather what is most properly fitting for the thing.
Think of attending a wedding. The request to attend is essentially for you—your person—to attend, but you perfect yourself by adorning yourself with the proper clothing fitting for being in attendance at a wedding. In this way, you and your appearance in that context are fittingly closely connected.
This is the sort of quality that, in my view, the gospel inaugurated. A Christian civil society, for example, is one that conforms to natural principles but adorns itself with Christian culture and worship. By doing so, both people and their culture in the context of God’s world are fittingly closely connected and thereby perfected. Christianity adorns the natural order without replacing or undermining it.
Evangelicals in their gospel-as-adjective talk usually fail to place their gospel terms in proper perspective. The Gospel is adjectival with regard to civil matters, but not in such a simple way. These additional duties must relate either positively or indifferently to the prior and immutable natural duties. It is better, in my view, to see the gospel as adjective only or mainly with regard to its perfecting of civil life. This means that Christians must necessarily affirm what is natural and seek to perfect the natural without undermining it.
 Francis Turretin on four ways to take the word “natural”:
“Natural is taken in four ways: (1) originally and subjectively, drawn from nature and concreated or born together with it and most deeply implanted in it (which is opposed to the adventitious); (2) constitutively and consecutively, constituting the nature of the thing or following and flowing from the principles of nature (as such as are the essential part or properties of a thing which is opposed to the accidental); (3) perfectively, agreeing with the nature and adorning and perfecting it (opposed to that which is against nature); (4) transitively, which ought to be propagated with nature.” Institutes 5.11.2. And: “It is one thing to speak of the essence of man; another of his integrity and perfection. At the taking away of a part or of some essential property, there follow in truth the destruction of the thing, but not forthwith at the privation of that which contributes to the integrity and perfecting of nature (as such as original righteousness was). The nature indeed remains mutilated and depraved (since it has lost what perfected it), but is not destroyed as to essence.” (5.11.11)