Philosophy of Religion (Loosely based on David Stewart's text)
Arguments for God's Existence
1. Anselm of Canterbury made the ontological argument for God's existence famous: subsequent versions were posited by Rene Descartes, Kurt Gödel and by Alvin Plantinga. The starting point for Anselm's ontological argument is God's being or perfect being theology. This form of argumentation is a priori because it starts from the concept of God (a perfect being in the absolute sense). We might also consider the ontological argument to begin with premises that are deductive and that logically proceed from a possible divine being to an actual divine being (i.e., God).
2. Existence is a great-making property or perfection for Anselm and Descartes: they reason that existent beings take precedence over merely possible entities. The Anselmian and Cartesian form of argumention is a priori as well since it begins with a particular idea concerning God's essence, namely, that God necessarily exists.
3. Why the focus on divine being when formulating the ontological argument? God is called Ἐγώ εἰμι ὁ ὤν (I am the being) in Exodus 3:14 (LXX). The Latin Vulgate has "ego sum qui sum" and also refers to God as "qui est." Both ways of treating the Exodus text seem to place emphasis on God's being: maybe 3:14 even identifies God as being itself (ipsum esse subsistens).
4. The medieval thinkers also tended to view essence and existence as distinct in relation to creatures but they argued that essence and existence in God's case are identical (the same thing), an idea known as absolute divine simplicity. So God exists necessarily because God is his own existence.
5. Descartes maintains that the very idea of God is "clear and distinct." Clear and distinct ideas are transparent, not obscure, self-evident, and easily distinguished from other ideas. The Euclidean postulate, "all right angles are equal to one another" is a clear and distinct idea. Another example is "all triangles are three-sided polygons." That proposition is clear and distinct like 2 + 2 = 4 is. What about the idea of God. Should it likewise be categorized as a clear and distinct idea?
To learn more about ontological arguments in general, see https://plato.stanford.edu/entries/ontological-arguments/
See David Stewart, Exploring the Philosophy of Religion, Seventh Edition (London and Toronto: Prentice Hall, 2010), ISBN: 978-0-205-64519-0.