Anselm is known for his *Proslogion* which argues for the existence of God.

Medieval Sourcebook: Anselm (1033-1109): Proslogium (fordham.edu) In this brief work the author aims at proving in a single argument the existence of God, and whatsoever we believe of God. –The difficulty of the task. –The author writes in the person of one who contemplates God, and seeks to understand what he believes. To this work he had given this title: Faith Seeking Understanding. He finally named it Proslogium, –that is, A Discourse.

Proslogion (Wikipedia) The Proslogion (Latin: Proslogium, lit. ‘Discourse’) is a prayer (or meditation), written by the medieval cleric Saint Anselm of Canterbury in 1077–1078, serving to reflect on the attributes of God in order to explain how God can possess seemingly contradictory qualities. This meditation is considered to be the first-known philosophical formulation that sets out the ontological argument for the existence of God.

Anselm is known for his Proslogion which argues for the existence of God.