The existence of God has been a question since the idea of God was conceived. Descartes tries to prove Gods existence, to disprove his Evil demon theory, and to show that there is without a doubt something external to ones own existence. He is looking for a definite certainty, a foundation for which he can base all of his beliefs and know for a fact that they are true. Descartes overall project is to find a definite certainty on which he can base all his knowledge and beliefs. A foundation that he will be able to prove without a doubt.
To find a definite certainty he uses a methodical doubt, this states that anything that could be doubted must be taken as false. This is done to find an absolute certainty for which to base all knowledge. Descartes develops his Evil Genius theory to bring doubt to all senses and hence making them false. The theory says that it is possible that there is someone (an evil genius) who is controlling our mind. If so we wouldn't be sure whether or not what we see was real or just a dream, therefore all of our senses would be doubtable and thus taken as false. This theory proves that nothing external is certain and must be proven false to continue his quest for truth and certainty.
Descartes second meditation gives a definite certainty for which to use as his foundation to build his beliefs. Then his third meditation proves the existence of God and the external world, while disproving the evil genius theory. That definite certainty that cannot be doubted, is the fact that I exist. Whenever I utter or think "I am" I know this to be absolutely true, without doubt. From this definite foundation Descartes tries to prove that there is something external to the mind.
So he states the law of casualty. This basically says that nothing can be created from nothing, and that the less perfect can not create something more perfect or better than itself. Then if there is an idea in our minds that we didn't create, something else created it. If God is more perfect than us, then we could not have created God but God created us. Descartes then wrote about the idea of God. He said that God is infinite and could not have been created by us because God is more perfect than us thus un doubtable and certain.
The idea that God exists disproves the Evil Genius theory therefore proves the existence of an external world. Anslem and Descartes' theories both prove that there is a God and they both use some similar ideas in their premises'. For instance, they both rely on the faith that there is an idea of God; he is perfect and self evident. God could not have been conceived if there wasn't a God.
Though, Descartes says that the less perfect can not create something more perfect or better than itself, and that we are less perfect than God therefore we could not have created God. So, this proves the existence of God and an external world. On the other hand, Anslem says Existence is greater than conception and nothing greater can be conceived then God. Therefore God must exist. In my opinion descatres has successfully proven the existence of God as well as one can because he started from the beginning and logically found a true foundation of beliefs and from there proved the existence of God and an external world.
consequently making our senses less doubtable, and more certain.