Today, 900 years ago exactly, Anselm of Canterbury died in 1109. He is ranked as one of the greatest theological minds in church history along with giants like Augustine and Thomas Aquinas. Here is a link to the Wikipedia article:
Anselm of Canterbury
He was a fascinating man. I am going to highlight just a few of his contributions to the Christian faith.
His first work of note was a work of philosophy and apologetics, not theology proper. Proslogion is the name it usually goes by but an English translation of the title is Discourse on the Existence of God. In this work Anselm developments his famous (or infamous, depending on one's worldview) ontological argument which, in a much simplified form, states that a being (i.e. God) other than which nothing greater can be conceived must necessarily exist; and since the Christian God is the greatest being possible, He must necessarily exist.
Theologians and philosophers still debate about the merits or dis-merits of the ontological argument for the existence of God. Even after 900 years Anselm's ideas are still alive and kicking.
His major theological work, and arguably his most important, especially from a theological perspective, is Cur Deus Homo. In English this can be variously translated as Why the God-man? or Why Did God Become Man? In it Anselm argues that the main reason why God the Son became man was to pay a debt, not to Satan as many early proponents of the "ransom theory" thought, but to God himself. He argued that in sinning against God humanity was morally required to make satisfaction to God's honor and holiness, and this satisfaction is exactly why hell is necessary. But Jesus provides salvation because he is the God-man; fully human because the satisfaction required is due from humanity, and fully God because the debt against an infinitely holy God is infinite and is therefore only something God can possibly pay in full.
The reason I have started this series on Christians who have gone before us is because I believe that history is an important part of knowing who we are and who God is. The Gospel is historical because it is about God's acts in history; specifically the life, death, and resurrection of Jesus. The Church is also historical, and ignorance of it's history is ignorance of where we came from and ultimately where we are going. I hope reading about Anselm of Canterbury is proof of that.