1). God is a necessary being.
You are not a necessary being. The world will get along perfectly well without you. You can either exist, or not exist, and the world will continue on. God is not such a being, as will be explained a bit later.
2). God as so defined either exists or does not exist.
According to the definition provided by the first point, God either exists, or doesn't exist.
3). If God exists, he necessarily exists.
If God exists, he has to exist.
4). If God does not exist, he necessarily does not exist.
If God does not exist, he cannot exist.
5). The idea of God is not self-contradictory.
The idea of God has no contradictions in it. It is not impossible for God to exist.
6). God exists.
Since it is not impossible for God to exist, and since if God does not exist, he cannot exist, God exists.
Discuss.