Classical theology is most commonly used to describe the religious beliefs and doctrines that have been traditionally associated with mainstream Christianity in the Western world and that have been regarded by many Christian churches and theologians as authoritative. Like other theologies, classical theology contains a systematic set of beliefs and arguments about religious questions such as the nature of God. The term is also sometimes used in reference to Judaism and Islam to refer to the traditional core beliefs of those religions, but in a Western context “classical theology” without any additional modifiers usually means traditional Christian theology.
The term "classical theology" was largely defined in the first few centuries of Christianity through councils of bishops such as the Council of Nicea in 324 AD and the writings of early Christian theologians such as Saint Ambrose of Milan, Saint Augustine of Hippo, and Saint Irenaeus of Lyons. Important later theologians in this school of thought include Saint Thomas Aquinas and Saint Anselm of Canterbury. It remains enormously important in Christianity in the world today and is the primary influence on the doctrines accepted by the Orthodox and Catholic churches as well as most major Protestant denominations.
Classical theology conceives of God as a perfect being who is omnipotent, omniscient, and omnibenevolent. God is regarded as eternal, meaning that he has always existed and has no beginning in time. Unlike created entities such as humans or the universe as a whole, his existence is not caused by anything else, a property called aseity. As God in classical theology is both perfect and not caused by or dependent on anything else, he is eternally unchanging, or immutable.
The classical view also sees God as a necessary being, meaning that God is a being that could not have failed to exist; there is no possible world in which God would not exist. This distinguishes God's existence from the contingent existence of created beings, such as humans, because the existence of any particular being or beings other than God is caused by a particular set of circumstances that could have been otherwise. Everything in existence other than God is regarded as dependent on God, as the only necessary and uncreated being, for its existence.
God's independence from external causation is also commonly taken to imply that God is also impassible, meaning that he does not experience feelings such as pleasure or pain from the actions of of other beings. Theologians who take this view argue that Biblical references to God being angered, pleased, or the like are not meant literally, much in the way that reference to God's hand or throne do not refer to physical body parts or a literal chair that God sits on. Instead, they are treated as metaphors for things beyond human understanding rather than a description of God experiencing changing emotional states caused by external events in the way a human does.
Classical Christian theology regards God as a single being who is a trinity of three equal persons called the Father, the Son, and the Holy Spirit. These persons are believed to be distinct and yet united as a single being, with all three persons having the same divine attributes described above. All three are perfect, omnipotent, and eternal. The precise nature of the trinity has been one of the most intensely controversial theological questions in the history of Christianity. Alternate views include unitarianism, the belief that God is a single person, and modalism, the belief that the persons who make up the trinity are all equally divine but are aspects, manifestations, or modes of God instead of discrete persons. In the Mormon faith, the trinity is considered to be not only three persons, but three distinct beings who are united in purpose but are separate entities.
Jesus Christ, in the classical view, is the incarnation of God the Son, who had both a fully human and a fully divine nature joined in one person. His suffering and death by crucifixion is believed to have served as an atonement for human sin, making salvation and reconciliation with God possible for humans. Jesus was able to suffer during the crucifixion because he possessed a fully human nature, even though his divine nature remained impassable. Historically significant rivals to this view include Arianism, the belief that Jesus was divine but was a created being subordinate to God the Father rather than co-eternal with him, and monophysitism, the belief that while Jesus had a human body he had only a single divine nature.