Final answer:
The concept that God is the Necessary Being necessary for the existence of everything else, as argued by St. Thomas, is called monotheism, which is the belief in one supreme God. This is aligned with Aquinas' philosophical arguments that establish God as the necessary existence to prevent infinite regress in causation and dependency.
Step-by-step explanation:
The concept that St. Thomas argues, where God is thought of as the Necessary Being, and His existence is necessary for the existence of everything else, is known as monotheism. This is a form of theism that asserts that there is only one God who is the supreme being and the fundamental cause of all that exists. Aquinas' Five Ways, which attempts to demonstrate the existence of God, refers to God as the necessary being that prevents an infinite regress in explaining existence. Aquinas uses the concept of necessary being to argue that while all things that exist are either contingent or necessary, there must be at least one necessary being upon which all contingent beings depend, and this being is what we commonly refer to as God. This is part of natural theology, which does not rely on the authority of the religious institution but on reason and observation of the world.