Moral necessity is necessity as applied to the activity of free beings. We know that men under certain circumstances, although they are free, will act in such and such a way. This moral necessity is the basis of moral certitude in historical and moral sciences. The term is also used with reference to freedom of the will to denote any undue physical or moral influence that might prevent the will from freely choosing to act or not act, to choose one thing in preference to another. In theology the notion of necessity is sometimes applied with special meaning.
Theologians divide necessity into absolute and moral.
A thing is said to be absolutely necessary when without it a certain end cannot possibly be reached. Thus revelation is absolutely necessary for man to know the mysteries of faith, and grace to perform any supernatural act. Something is said to be morally necessary when a certain end could, absolutely speaking, be reached without it, but cannot actually and properly be reached without it, under existing conditions. Again, in relation to the means necessary to salvation theologians divide necessity into necessity of means and necessity of precept. Thus faith and baptism of water are necessary by a necessity of means, the former absolutely, the latter relatively, for salvation.