A few things to note about the argument.
- The ontological argument is supposed to prove that God exists from the mere definition of God as the greatest conceivable being.
- It is an a priori argument.
- First put forward by St. Anselm.
- The most famous objector to this argument was Kant who argued that Existence is not a predicate i.e. to say that God exists adds nothing to the definition of God.
One simple version of the ontological argument (there are lots of types and versions of the ontological argument, see the SEP entry below for more on that):
1. God is a being which has every perfection. (This is true as a matter of definition.)
2. Existence is a perfection.
3. So, God exists.
The Problem: The conclusion doesn’t actually follow from the premises. What follows is that existence is part of the definition of God, but not that he actually exists.
Take a look at the SEP entry below. At least read the introduction, which you will be responsible for knowing. Try to take a look at the rest or at least part of the rest of the article, but don’t lose too much sleep over it.
Introduction to Philosophy 