When I taught philosophy of religion at Monroe County Community College (18 years) I would open with Anselm's Ontological Argument for God's Existence. (See here.) The argument goes:
1. I have an idea of a being a greater than which cannot be thought.
2. Therefore, God exists.
Note that premise one is not about a really great being. It's about a greatest possible being; viz., a being a greater than which cannot be logically conceived.
Occasionally, I would introduce students to Alvin Plantinga's Modal Version of the Ontological Argument for God's Existence. One form of which, is this.
1. It is possible that a logically necessary being exists.
2. Therefore, God exists.
Occasionally, I continue reading about this fascinating argument.
See the recent edition of Philosophy Now - "The Ontological Argument Revisited."
