An Argument from the Definition of "God" and the Possibility That God Exists
Alvin Plantinga defines God as, together with the standard attributes, a "necessary being." A necessary being, if one existed at all, would exist no matter how the world went (it would exist in every "possible world"). If one admits that it is possible that God exists (as most will), then it follows that God does exist:
in order for it to be possible at all, there must be at least some possible world in which it will be true that God exists (this is, philosophers argue, just what it means to say that something is "possible"); but (and here is the tricky part), if there is some world where it is true that God exists, then "God exists" must be true in every world. Why? Because the very definition of God demands that God is a necessarily existing being. Thus, we are forced to admit that if we "find" him existing in even one world, we are guaranteed (by the definition) that he must be found in every other possible world as well. Thus, God is found (i.e., exists) in this world.Greatest Strength: Most opponents of theism are willing to concede that God's existence is at least possible.
Greatest Weakness: Atheists who have considered the argument can and do reject the claim that God is even possible. They argue instead that the very concept of God is incoherent since, for example, it entails paradoxes such as the paradox of the stone: Can God make a stone so big that he cannot lift it? This is supposed to show that an omnipotent being is impossible.1
1 William C. Davis, Reason for the Hope Within (Grand Rapids, MI: Eerdman's Publishing Co., 1999), p. 25.