It seems English A level may prove to be pretty useful for this... Descartes' Ontological Argument.

Firstly, something that is worth noting about Descartes, is that as a sceptic, he approached his arguments from the point of view that he is proving that God cannot be doubted. This was because he always worked towards an end goal, by starting off by doubting everything at first. Which is where his famous quote "cogito ergo sum" or "I think therefore I am" came from, as in this case he used scepticism to make a point about whether he even existed himself.
  

So what did the dubbed 'father of modern philosophy' have to say about God? Well his argument is as follows...  Firstly alternatively to Anselm's description of God as that than which nothing greater can be conceived, he says God is the supremely perfect being. So by stating this at the beginning, it already shows God to be a being, and therefore negating the need to change him from a concept or an idea into a being at a later stage in the argument. His second point is that a supremely perfect being has all the perfections that there can be. Indeed by giving God attributes, it is therefore enforcing this idea of God as a being, rather than a whishy washy concept or idea. Thirdly, Descartes says that existence is a perfection, because similarly to Anselm's argument, being in existence is better than not, therefore making it a perfection. And lastly to end his argument he says therefore God exists necessarily. Now an alternative way that he could have ended his argument which would have saved it from contradicting itself, would have been to say therefore he possesses the quality of existence. This would have ensured that he did not confuse God to exist (verb), and God to have existence, (abstract noun), and therefore not have stated him to have two differing attributes. The problem within these arguments, is that when people such as Descartes use both the verb and the abstract noun to describe God, it is just completely incoherent. For, the definition a verb is a doing word, so it would not make sense to say that God has 'exist'. Which is what he starts the argument off by saying, but by replacing the word exist with existence. So again, if he had altered the last point of his argument it would be coherent, as it would just show God to possess existence.

Despite this, another part of his argument that is clear to us, is that he sees existence as something that God has, which makes existence a predicate to God. This means that it is a quality that God possesses and affirms it as an attribute of him. He gives the example of triangles, and mountains. In these examples he says that you can think of a triangle only as a shape that has 3 interior angles that add up to 180 degrees. And a mountain can only exist if there is a valley along side it as well. However, although it is true that you can imagine these two things not actually existing within reality, it is impossible to think of God as not existing, as he is entwined with existence like a triangle is with having three angles.