Ontological arguments feature prominently in the history of Christian philosophy. An ontological argument is, roughly, one that tries to establish the existence of God from God’s nature, or definition which captures that nature. The aim of this paper is not to present a survey of such arguments. Rather, the point is to home in on what I take to be the central nerve of such arguments: the Characterization Principle – essentially, a principle to the effect that an object has those properties it is characterised as having. The principle interacts in important ways with two other notions: existence and necessity. They will also, therefore, fall within the ambit of the discussion. We will analyse matters by looking at ontological arguments as presented at various historical times. The earliest ontological argument for a Christian god was given by Anselm of Canterbury. We come to him in due course. We will start with early modern philosophy, where the nerve of the argument is at its most exposed. We will then turn back to Anselm. After that, we will move on to later modern philosophy.