Id quo maius non cogitari potest. That than which a greater cannot be thought.
1078 Of God. Proslogion, ch.3. His famous ontological argument for the existence of God. Since a being that exists is necessarily greater than a being that does not, God must by this definition exist.
Neque enim quaero ut credem, sed credo ut intelligam. For I do not seek to understand so that I may believe; but I believe so that I may understand.St Anselm