This is an excerpt from Philosophy of Religion by C. Stephen Evans in the chapter for classical arguments for God's Existence. In this excerpt, Evans describes Norman Malcolm's formulation of Anselm's second ontological argument, which deals with the concept of necessary existence.
The gist of the second argument, as Malcolm formulates it, is as follows: God is by definition a being who does not merely happen to exist. God can neither come into existence nor pass out of existence, since a being who could do either simply would not be God. It follows from this that if God exists at all, then his existence is necessary. If he does not exist, then his existence is impossible. But either God exists or he does not exist, so God's existence is either necessary or impossible. Since it does not seem plausible to say that God's existence is impossible, then it follows that his existence is necessary. So if God's existence is possible, then it is necessary. More formally the argument can be put like this:
1. If God exists, his existence is necessary.
2. If God does not exist, his existence is impossible.
3. Either God exists or he does not exist.
4. God's existence is either necessary or impossible.
5. God's existence is possible (it is not impossible).
6. Therefore God's existence is necessary.1
1 C. Stephen Evans, Philosophy of Religion: Thinking About Faith (Downers Grove, IL: InterVarsity Press., 1982), p. 48.