This is a response to the question of why "that than which nothing greater can exist" is automatically identified with God. It seemed like a good summary for the whole shabang at this point, the important parts at least...

It's not that "That than which nothing greater can be thought" reminds him of God or makes him think 'oh, that would be God', it's that that is what god is, he is proving to the mind with no previous conviction that there is a 'God', so instead of "you know God, well he's that than which nothing greater can exist", it's really:

given the human mind and it's ability to extract and create concepts of good and just and happy we can construct an idea that is the absoluteness of each of those qualities combined, that is, we know happiness so we can extrapolate the possibility of absolute happiness - the same goes for justness, goodness, etc (anything that we consider to be 'good') - thus when we take the ideas of these absolute concepts (absolute happiness, absolute goodness, etc) and combine them into one thing (after all if many separate things are deemed good it follows that together they are better because they are all at once, so to speak) we come to the idea of "that than which nothing greater can be thought", Anselm refers, as the whole of christian culture does, to this wholly perfect and good thing as God. We can see now that God is not identified with the idea of "that than which nothing greater can be thought", but rather that idea is the pretense of understanding what God is; to Anselm God is defined by this statement, and says that he exists solely because of this property.

This is known as an analytic (analytic statements will be a very important part of your philosophic basis of knowledge, and it's a fancy term to whip out in conversation once you get the hang of terms surrounding it that you'll get to soon enough) statement, which is a statement that is true simply by the nature of the subjects meaning, and that the predicate adds no further information to the subject. A simple example being: a bachelor is unmarried. Bachelor being the subject, which directly refers to an unmarried male, and unmarried being the predicate, which merely defines bachelor and adds nothing new to the term and idea of bachelor. Anselm believes that God is known analytically, that he exists purely by definition:

Now that we see that this idea of "that than which nothing greater can be thought" can exist in the understanding - we understand that we can extrapolate the idea of happiness into perfection and do such with every other goodness we know and combine them - Anselm says that God exists analytically, or by definition of this statement. First, we must agree that something in understanding is not as great as something in reality - imagine the perfect weekend, perfect sex, etc, thinking about it is not as good as the reality of it. So, says Anselm, God must exist because "that than which nothing greater can exist" as an idea, which we've accepted as being understood in our minds, has the property of existence because we also understand that existence is greater than something only in the understanding (sex better than thinking about sex); thus "that than which nothing greater can be thought", or God as we like to call it, must exist. Furthermore "that than which nothing greater can be thought" cannot be thought not to exist, because if it is thought not to exist it is then a thought of something that is not "that than which nothing greater can be thought", because that thought, as we've deduced, encompasses the property of existence. The language here is a little bit of a pain in the ass: all he's saying is that when you think of something than which a greater cannot be thought a necessary component of that thought is existence, because once the existence of that thought is doubted it ceases to be the thought of that than which nothing greater can be thought because a greater can be thought, that greater thing being an existing thing.