Necessary being' is one of the terms by means of which Christian thought has sought to define the difference between God and man. The notion of necessary being, applied to God and withheld from man, indicates that God and man differ not merely in the characteristics which they possess but more fundamentally, in their modes of being, or in the fact that they exist in different senses of the word ‘exist’.
That such a distinction, however it may be best expressed, is essential to the Christian concept of God is agreed virtually on all hands. Paul Tillich in our own day emphasises the distinction to the extent of using different terms to refer to the reality of God and of man respectively. Human beings and other created things exist; God, on the other hand, does not exist, but is Being-itself. This is the most recent way of formulating a discrimination which has been classically expressed in the history of Christian thought by the idea of the necessary being of God in contrast to the contingent being of man and of the whole created order. There are, however, two importantly different concepts which may be, and which have been, expressed by the phrase ‘necessary being’. ‘Necessity’, in a philosophical context, usually means logical necessity, and gives rise in theology to the concept of a being such that it is logically impossible that this being should not exist. But this is not the only kind of necessity referred to in philosophical literature.