The writings of Anselm are not easy to understand. His scholastic approach to reason is highly syllogistic, which can make for some complex arguments that are not always reader friendly. In fact, the last work of his main writings is a response to Gaunilo, who he argues has misunderstood the arguments of the Proslogion.
Another difficulty in understanding Anselm is that his writing was done in an era of Christian thought that would seem foreign to those who have not studied it. Christianity during the eleventh century was still highly mystical. That means that Anselm's ideas that humans can use their minds to know God would have been highly controversial, even though to the modern Christian, many of his ideas seem obvious and normal.
Among his contributions is his belief that Jesus's death was 'satisfactory.' That doesn't mean that God was pleased to sacrifice Jesus, but rather that a debt was 'satisfied' in the crucifixion. Another way to say this is that God's wrath was incurred as punishment for human sin, and Jesus's suffering on the cross was him taking that wrath upon himself, in order to settle the debts of those who believe in him. The other way to view the crucifixion of Christ is that Jesus allowed himself to be killed and resurrected as a metaphor for human salvation. This school of theology is called Soteriology, and it deals with the nature of sin, the nature of condemnation and the manner by which a Christian is saved from death.
Another important contribution was Anselm's bold argument that even an atheist, if he works through the reasoning with an honest heart, will quickly discover the existence of God. In fact, Anselm argues that any person of even moderate intelligence should be able to conclude that God exists. His Ontological argument, therefore, although it concludes in the argument for the existence of God (Ontology means the study of existence), is a defense not only of God's existence, but for the existence of the Christian God in particular, a God who loves the humans and embodies moral goodness.
This leads into another of Anselm's contributions, that a man's understanding of moral goodness (that man recognizes some things are right and some things are wrong) is an indication of higher moral principles, and therefore God's goodness is the form of objective goodness. This means that through the contemplation of what is truly 'good,' man should be able to draw conclusions about God's personality and nature, eventually 'knowing God.'
