Norman Malcolm's Ontological Argument

This argument for me at first glance, was perhaps the most easy to read, and understand. Simply due to the fact that unlike Descartes and Anselm's arguments there is no confusing definition of God to wrap my head around. Furthermore, because he was using parts of Anselm's Proslogian to form his argument (chapters 2 + 3), it could be assumed that he also thought of God as that than which nothing greater can be conceived. However, because he, like Kant saw error within chapter 2 of Anselm's Proslogian, he sought to use chapter 3, which he established as a second ontological argument which was not susceptible to such criticism as a way to form his own argument.

So, on to his argument, which is as follows;

1) If God does not exist, his existence is impossible
2) If God does exist, then his existence is necessary
3) God's existence is either impossible or necessary
4) God's existence is not impossible
5) Therefore God's existence is necessary

Now, from this argument a few things can be identified, the first thing being that because of the implicitly assumed definition of God, Malcolm therefore states that God is immutable or unchanging. This means that God couldn't and wouldn't change from non-existence to existence, because as he says 'If God does not exist, his existence is impossible'. Another key point about this argument is how Malcolm gives an example of the only thing that is truly impossible, which is the statement "Nothing Exists" as this is an oxymoron, or a contradiction. And so secondly due to this, a further thing that this argument shows is that because of that oxymoron, this next statement "God Exists" can be possibly true and is therefore why he says that the existence of God is not impossible.
Thirdly and finally, another side that can be drawn from his argument, is that of the four possibilities concerning God, which are;

1) Necessarily false - God can't exist.  (any statement that were to fall under this are contradictory propositions, such as the statement 'this square is round)

2) Contingently false - Could but doesn't exist.
(the reasons that 2 and 3 cannot be applied to God are simply because God shouldn't be limited or dependant upon anything else)
3) Contingently true - Could and does exist.

4) Necessarily true - Has to exist. 

So overall, the reason why God is being proved to exist in Malcolm's argument is because God cannot be brought into existence, or be made to stop existing, therefore necessitating God's own existence. And in addition to this because the existence of God is not impossible, it means that there is a chance of this necessary God existing.