In classical apologetics there are four common philosophical arguments for the existence of God. The first argument is entirely cognitive, the ontological argument. The cosmological and teleological arguments formulate their first premises from nature. The fourth argument does not rely upon reason or creation but conscience. The moral argument says that because everyone has a perception of right and wrong, there must be One who has created in all humanity an understanding of morality.
The moral argument has the briefest formulation of all the philosophical arguments. An objective morality exists in this world. The best explanation for this objective standard is God. Therefore, God exists. A variation of this argument is seen in the existence of measures of goodness. Humanity makes determinations about the goodness and excellence of things. Since there exists the ability to measure excellence, there must be that which is of highest excellence. This highest good is God.
The most common expression starts with the recognition that generally all human populations hold to similar standards of right and wrong. For example prohibitions against murder, theft and deception are common. Even in those situations where a people group holds a different standard of right and wrong, there is still a recognition of some standard of right wrong. Because everyone knows some things are right and some things are wrong there must be a universal basis for the existence of the human conscience. The best explanation as the basis of the conscience is God.
Natural explanations for morality struggle to explain the conscience of man, especially the existence of an altruistic standard. The selfless sacrifice for the benefit of others doesn’t quite fit in a system in which a species is best helped by the fittest members surviving to pass on their genes. Altruism is undeniably common in humanity and can often be found in the animal kingdom. Naturalists have offered varying explanation for the existence of altruism. Many of them offer models by which altruism benefits the survival of the species. For many observers the naturalistic explanations of altruism are not very convincing. A Divine, moral being best explains the existence of selfless love found throughout much of humanity.
Romans 2 appeals to the moral argument. Paul says when those who do not know the law of God do those things commanded in the law then they show the law of God is written in their hearts. However, Paul does not use this argument as proof of the existence of God. Paul presupposes the existence of God and the existence of an objective moral standard. Paul instead uses the moral argument to show the guilt of every person. Because everyone has a conception of right and wrong none can stand before God ignorant of the law or innocent according to the law. Everyone has done that which violates their own conscience and, more importantly, which violates the absoluate standard of God’s law. This leads to the inevitable conviction, “There is none righteous”. Because all are guilty before God the only hope for men is forgiveness of sin through faith in Jesus.