Anselm of Canterbury, known also as St. Anselm or Anselme de Buc, was an 11th century abbot whose writings have been traditionally heralded for their logical and philosophical basis. During a time when many theologians were writing from a more mystical perspective, Anselm brought an approach that was logical and skeptical of mystic reasoning. For this, he has often been called the father of modern scholasticism, especially as the term applies to religion and religious philosophy.
The two most famous contributions Anselm made are the ontological argument for the existence of God and the idea that Jesus's death was an act of restitution, to settle the debt incurred by the sinfulness of man by suffering the wrath in their stead.
His major writings include the Monologion (1075), which is a treatise on the existence of God and the nature of faith and reason. Then, Anselm wrote another follow-up treatise called the Proslogion (1077) to discuss issues raised in the Monologion, but with a more strictly logical voice. It's this work that contains his famous ontological argument.
After the Proslogion, Anselm was refuted by others in the scholarly community, including an important work by Gaunilo which he called, For the Fool, which prompted Anselm to rebut in his work the Responsio.
Other major works by Anselm include Cur Deus Homo, which contains his argument for satisfactory atonement (the idea that Jesus died to pay the price of man's sin), De Fide Trinitatis et de Incarnatione Verbi Contra Blasphemias Ruzelini, and De Conceptu Virginali et de Originali Peccato, both dealing with major beliefs in the Christian faith, namely the Trinity, the incarnation, the virgin conception and original sin.