Aseity | Creator | Eternity | Goodness | Immutability | Impassibility | Ineffability | Necessity | Omnipotence | Omnipresence | Omniscience | Perfection | Trinity
God is thought by most theists to be a necessary being. To say that God is necessary is to say that his non-existence is impossible, that there is some contradiction in the thought that God might not exist.
Contemporary philosophers tend to think about possibility in terms of possible worlds. Any world that can be described without contradiction is a possible world, whether it actually exists or not. The pop-scientific view according to which whenever we make a decision the other alternative is played out in some alternative dimension thus involves different possible worlds coming into being whenever we are faced with a choice. Each of the alternate dimensions is another reality, another possible world. Of course, it is a possible world whether it comes into being or not; as long as it can be described without contradiction, it is possible.
To say that something is necessary is to say that it exists in all possible worlds. If God is a necessary being, then in every one of those world that can be consistently described, God exists.
Godís necessity has implications for natural theology, the project of discovering God by examining the natural world. If God exists in all possible worlds, then there might be no need to study the world that we are in, looking for empirical evidence to support the existence of God, in order to discover whether we are in a possible world where God exists or a possible world where he doesnít. If God exists in all possible worlds, then no matter which possible world is actual, God exists. Many necessary truths can be discovered simply by reasoning about them, independent of any empirical investigation. One of the classical arguments for Godís existence, the ontological argument, uses Godís necessity to prove his existence in this way.