In writing about Christian attitudes toward justice, Jacques Ellul argues that natural law is incompatible with justification by faith:
Such a theory [of Natural Law] leads to an elimination of the Doctrine of Justification, for this doctrine when properly understood puts an end to every pretension of Man to know Justice naturally by his own means --- to say nothing of achieving it. If God alone is just and does not impart his righteousness to Man except in justification --- if He does not make known to Man His work of justice except by revealing Himself to Man --- then we must conclude that Man knows nothing of justice apart from this act of God; that there is no justice written in the nature of Man.
In other words, how can there be justice (righteousness) in nature when we are wholly dependent on God for righteousness? It is an interesting point. I know that many Christians partake of both sides of the dichotomy which Ellul is presenting here.