Saint Anselm of Canterbury (1033-1109) was a Benedictine Monk and philosophical theologian who eventually became Archbishop of Canterbury. He is best known for his approach to faith and reason captured in the phrase “faith seeking understanding” (fides quaerens intellectum); his ontological argument for God’s existence (found in his Proslogion); and his articulation of the satisfaction theory of the atonement.
Some have misunderstood “faith seeking understanding” to mean that Anselm valued knowledge or evidence higher than faith or belief, but
Faith for Anselm is more a volitional state than an epistemic state: it is love for God and a drive to act as God wills. . . . So “faith seeking understanding” means something like “an active love of God seeking a deeper knowledge of God.”
Anselm’s ontological argument (the designation “ontological” comes from Kant) is an argument that begins by describing God as “that than which nothing greater can be thought.” Anselm argues that if this definition of God exists in the understanding, then God must exist, since a being who met this criteria but failed to exist would not be as great as a being who met this criteria and did exist. As a result, if God is a possible being, then He is also a necessary being, and necessarily exists.
Versions of this argument have been defended by Rene Descartes, Gottfried Leibniz, Charles Hartshorne, and Alvin Plantinga. Critics of the argument include David Hume, Immanuel Kant, and Gottlob Frege.