The nature of God is paramount in apologetics, so today we will continue to explore what that nature entails. The article below explains the necessity of God:
Necessity (Noncontingency). God is by nature an absolutely necessary Being. That is, he cannot not exist. God is not a may-be but a must-be kind of Being. He is not contingent, since he does not have the possibility not to exist. If he has no potentiality not to exist, then he must exist.
This is not to say that the ontological argument is valid. Aquinas considered and rejected Anselm’s proof for God. If God (i.e., Pure Actuality) exists, then he must exist necessarily. But one cannot simply define him into existence. Aquinas offered his famous cosmological arguments for God’s existence (Summa Theologica, 1.2.3). And once we know, from reason and revelation, that God exists, then we can be sure that he must exist necessarily. Such a Being has no potential not to exist.